Optical communication apparatus and optical add/drop apparatus

ABSTRACT

The present invention relates to an optical communication apparatus for detecting the intensity of light in an optical modulator or the intensity of a modulation signal and for controlling the operating point of the optical modulator based on the result of detection. In the optical communication apparatus having such a configuration, the operating point of the optical modulator can be kept stable even when the input light or the modulation signal is temporarily non-existent in the optical communication apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention, an optical communication apparatus whichtransmits wavelength-division multiplexed signal light, relates to anoptical communication apparatus whose operation is stabilizedirrespective of presence/absence of input light or a modulated signal tobe transmitted, as well as to an optical add/drop apparatus using suchan optical communication apparatus as an addition apparatus.

2. Description of the Related Art

Ultra-long-distance and large-capacity optical communication apparatusesare now required to construct future multimedia networks. Concentratedstudies are now being made of the wavelength-division multiplexing as amethod for realizing large-capacity apparatuses in view of suchadvantages that it can effectively utilize a wide bandwidth and a largecapacity of an optical fiber.

In particular, studies are in progress about an optical add/dropapparatuses of the wavelength-division multiplexing method and opticalmodulators used in the addition section of such an optical add/dropapparatus that is required in each node of the lightwave network.

In the Mach-Zehnder interferometer type optical modulators (hereinafterabbreviated as “MZ modulator”) that are used as optical modulators inconventional optical communication apparatuses, it is necessary tostabilize the output optical signal with respect to a variation and thevariation with temperature and time. Japanese Patent Laid-open No.251815/1991 discloses an operating point control circuit for controllingthe operating point of an MZ modulator intended for this purpose.

FIG. 20 is a block diagram of an MZ modulator having this conventionaloperating point control circuit.

As shown in FIG. 20, light exit from a light source 310 such as a laserdiode (hereinafter abbreviated as “LD”) is entered to an MZ modulator311. A modulation signal including information to be sent and alow-frequency signal of a predetermined frequency f0 that is outputtedfrom a low-frequency oscillator 324 are inputted to a variable gainamplifier 313. The variable gain amplifier 313 superimposes thelow-frequency signal of the predetermined frequency f0 on the modulationsignal and outputs it, which is then inputted to one modulation-inputterminal of the MZ modulator 311 via an amplifier 314 for obtaining apredetermined signal level and a coupling capacitor 315. A bias Tcircuit consisting of an inductor 316 and a capacitor 317 is connectedto the other modulation-input terminal of the MZ modulator 311. Thecapacitor 317 is grounded via resistor 318. A portion consisting of theamplifier 314, the coupling capacitor 315, the bias T circuit, and theresistor 318 are equivalent to a drive circuit of the MZ modulator 311.

The MZ modulator 311 modulates light that is supplied from the lightsource 310 with a signal that is given by the drive circuit and outputsa resulting signal.

Part of an optical output of the MZ modulator 311 is branched and takenout by an optical coupler 312. The branched part of the optical outputis detected by a photoelectric converter 319 such as a photodiode(hereinafter abbreviated as “PD”), and the detection signal is amplifiedby a buffer amplifier 320 that selectively amplifies a frequencycomponent of f0 and inputted to a multiplier 321. The low-frequencysignal that is outputted from the low-frequency oscillator 324 is alsoinputted to the multiplier 321. The multiplier 321 compares the phasesof the signal that is inputted from the buffer amplifier 320 and thelow-frequency signal that is inputted from the low-frequency oscillator324, and outputs a signal in accordance with a phase difference.

Therefore, the multiplier 321 can detect the low-frequency signal of thepredetermined frequency f0 that was superimposed by the variable gainamplifier 313.

An output signal of the multiplier 321 is inputted to one input terminalof a differential amplifier 323 via a low-pass filter (hereinafterabbreviated as “LPF”) 322 that allows passage of a frequency componentof the predetermined frequency f0 or less. On the other hand, the otherinput terminal of the differential amplifier 323 is grounded. An outputof the differential amplifier 323 is inputted to the inductor 316 of thebias T circuit as an error signal to be used for moving the operatingpoint of the MZ modulator 311, whereby the bias value is variablycontrolled so as to correct the operating point.

In the MZ modulator having the above configuration, the superimposedlow-frequency signal of the frequency f0 does not appear in the outputlight when the bias value is in the optimum state.

FIG. 21 is a waveform diagram showing an operation in a state that theoperating point drifts in the MZ modulator having the above circuitconfiguration. Part (a) of FIG. 21 shows input/output characteristics ofthe MZ modulator, in which curve B represents an input/outputcharacteristic in a case where the operating point has drifted to thehigh-voltage side from that of curve A and curve C represents a casewhere the operating point has drifted to the low-voltage side from thatof curve A. Part (b) of FIG. 21 shows a waveform of an input signal andparts (c), (c1), and (c2) of FIG. 21 show waveforms of output opticalsignals of the respective input/output characteristics.

As shown in FIG. 21, when the operating point has drifted to thehigh-voltage side or the low-voltage side, low-frequency signal of thefrequency f0 superimposed in output light appears with a phase that isinverted by 180° depending on the drift direction. Therefore, the biasvoltage can be controlled by using a signal coming from the multiplier321, whereby the drift of the operating point can be compensated for.

In this manner, a drift of the operating point can be compensated for bytaking out a low-frequency signal from output light that has beenproduced by modulating input light with a modulation signal and thelow-frequency signal and then comparing its phase with the phase of theoriginal low-frequency signal. Therefore, the operating point controlcircuit described above can control the operating point to stabilize itin a case where input light (output light) and a modulation signalexist.

FIG. 22 is a block diagram showing a conventional optical add/dropapparatus.

As shown in FIG. 22, after wavelength-division multiplexed signal lighttransmitting through an optical transmission line is amplified to apredetermined light intensity, it is then entered to an OADM (opticaladd-drop multiplexer) node section 350 which adds/drops on thewavelength-division multiplexed signal light. Signal light beams ofpredetermined wavelengths are dropped by the OADM node section 350 andsubjected to receiving operations in optical dropping sections 352 thatare provided in the same number of signal light beams to be branched byan optical coupler 351. Signal light beams to be added by the OADM nodesection 350 is generated by optical addition sections 355. The opticaladdition sections 355 are provided in the same number of signal lightbeams of respective wavelengths to be added by the OADM node section350. The added signal light beams and the signal light that has not beendropped in the OADM node section 350 are wavelength-divisionmultiplexed, amplified, and then outputted to the optical transmissionline.

In each optical addition section 355 of this optical add/drop apparatus,light that is exit from an LD 360 for generating light of a particularwavelength is amplified by an optical amplifier 361. Output light of theoptical amplifier 361 is modulated by an optical modulator 362 havingthe above-described operating point control circuit. The modulatedoptical signal is amplified by an optical amplifier 363 and then enteredto an optical coupler 354. The optical coupler 354 adds this opticalsignal to the OADM node section 350 together with optical signals ofother wavelengths that have been generated by other optical additionsections 355 having the same configuration.

Incidentally, in the MZ modulator 311 shown in FIG. 20, the followingproblem occurs when there is a short break in which the input lightentered to the MZ modulator 311 is temporarily non-existent and thenrecovers.

When the input light no longer exists, there is no light output to bebranched by the optical coupler 312 and hence the operating pointbecomes indefinite. That is, in part (b) of FIG. 21, it is impossible tojudge whether the bias voltage Vb is (1) 0 V or less, (2) greater than 0V and smaller than Vp, or (3) Vp or more.

If the input light recovers in such an indefinite state, in case (2) theoptimum operating point is established by the operation of the bias Tcircuit. However, the optimum operating point is not established incases (1) and (3); Vb is predetermined at 0 V in case (1) and Vb ispredetermined at Vp in case (3).

By these reasons, when a short break occurs in the input light that isincident on the MZ modulator 311, the optimum operating point is notnecessarily obtained.

Hitherto, the above problem did not occur because the MZ modulator 311was used in terminal stations or the like where no short breaks occur onthe input light. However, where the MZ modulator 311 is used in eachoptical addition section 355 of the optical add/drop apparatus of FIG.22, it is necessary to switch the wavelength of addition light to awavelength that is not used in a wavelength-division multiplexed signaltransmitting through the optical transmission line. This necessarilycauses, during such wavelength switching, a state where no input lightexists. Therefore, the solving the problem of being in the aboveindefinite state is a particularly important issue.

On the other hand, in the optical add/drop apparatus of FIG. 22, whenthere is no input light to the optical modulator 362, ASE (amplifiedspontaneous emission), which is a noise level spontaneously generated bythe optical amplifiers 361 and 363, is outputted to the opticaltransmission line. Further, each optical addition section 355 does notalways have a modulation signal to be added. When no such modulationsignal exists, not only ASE but also input light that is not modulatedwith any modulation signals are outputted to the optical transmissionline.

Further, in optical communication networks, the judgement ofmalfunctions occurring therein is based on the light intensity.Therefore, the malfunction cannot be judged if ASE or input light thatis not modulated with a modulation signal is inputted to an opticaltransmission line.

SUMMARY OF THE INVENTION

The first object of the present invention is to provide an opticalcommunication apparatus for keeping the operating point of an opticalmodulator stable even when the input light or the modulation signal istemporarily non-existent in the optical communication apparatus.

The object is attained by the optical communication apparatus fordetecting the light intensity of light in an optical modulator or theintensity of a modulation signal and for controlling the operating pointof the optical modulator based on the result of detection.

The second object of the present invention is to provide an opticalcommunication apparatus which does not exit ASE nor input light that isnot modulated with an modulation signal even when the input light or themodulation signal is temporarily non-existent in the opticalcommunication apparatus.

The object is attained by the optical communication apparatus fordetecting the intensity of light in an optical modulator or theintensity of a modulation signal and for regulating the intensity oflight transmitted to an optical transmission line from the opticalmodulator based on the result of detection.

The regulation of the light intensity is regulated, for example, by anoptical attenuator for attenuating the intensity of light entered to theoptical modulator or by an optical attenuator for attenuating theintensity of light exit from the optical modulator. As another example,switching the optical modulator regulates the regulation of the lightintensity.

As one of aspects of the present invention, the optical communicationapparatus can be used in an optical add/drop apparatus for adding anddropping an optical signal to and from wavelength-division multiplexedoptical signal.

As another aspect of the present invention, the optical add/dropapparatus, even when there are any unused communication apparatuses,since the input light or the output of the optical modulator of anunused addition apparatus and the modulation signal are monitored, theoperating point of the optical modulator is kept stable and neither ASEnor input light that is not modulated with a modulation signal is exitfrom such an addition apparatus.

Besides, another objects and characteristics of the present inventionwill be described specifically as follows referring to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the optical communication apparatusaccording to the first embodiment;

FIG. 2 is a block diagram of the optical communication apparatusaccording to the second embodiment;

FIG. 3 is a block diagram of the optical communication apparatusaccording to the third embodiment;

FIG. 4 is a block diagram of the optical communication apparatusaccording to the fourth embodiment;

FIG. 5 is a block diagram of the optical communication apparatusaccording to the fifth embodiment;

FIG. 6 is a block diagram of the optical communication apparatusaccording to the sixth embodiment;

FIG. 7 is a block diagram of the optical communication apparatusaccording to the seventh embodiment;

FIG. 8 is a block diagram of the optical communication apparatusaccording to the eighth embodiment;

FIG. 9 is a block diagram of the optical communication apparatusaccording to the ninth embodiment;

FIG. 10 is a block diagram of the optical communication apparatusaccording to the tenth embodiment;

FIG. 11 is a block diagram of the optical communication apparatusaccording to the eleventh embodiment;

FIG. 12 is a block diagram of the optical communication apparatusaccording to the twelfth embodiment;

FIG. 13 is a block diagram of the optical communication apparatusaccording to the thirteenth embodiment;

FIG. 14 is a block diagram of the optical add/drop apparatus accordingto the fourteenth embodiment;

FIG. 15 is a block diagram of the optical add/drop apparatus accordingto the fifteenth embodiment;

FIG. 16 is a block diagram of the optical add/drop apparatus accordingto the sixteenth embodiment;

FIG. 17 is a block diagram of the optical add/drop apparatus accordingto the seventeenth embodiment;

FIG. 18 is a block diagram of the optical add/drop apparatus accordingto the eighteenth embodiment;

FIG. 19 is a block diagram of the optical add/drop apparatus accordingto the nineteenth embodiment;

FIG. 20 is a block diagram of an MZ modulator having a conventionaloperating point control circuit;

FIG. 21 is a waveform diagram showing an operation in a state that theoperating point drifts; and

FIG. 22 is a block diagram of a conventional optical add/drop apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of optical communication apparatuses according to theinvention will be hereinafter described with reference to theaccompanying drawings. Besides, the same reference numerals of eachfigure indicate that they have the same constructions and descriptionthereof will be omitted in the following.

The optical communication apparatus according to the first embodimentwill be explained based on the accompanying figure.

In FIG. 1, this optical communication apparatus is composed of opticalbranching unit 10 and 12, an optical modulating unit 11, an operatingpoint controlling unit 13, a controlling unit 14, and an opticaldetecting unit 15.

Light entered to an input port is branched by the optical branching unit10 that branches light into two. The first branched input light that hasbeen branched off by the optical branching unit 10 is modulated by theoptical modulating unit 11 in accordance with a modulation signal to betransmitted. A modulated optical signal that is outputted from theoptical modulating unit 11 is branched by the optical branching unit 12that branches light into two.

The first optical signal branched off by the optical branching unit 12is exit to an output port. On the other hand, the second optical signalbranched off by the optical branching unit 12 is entered to theoperating point controlling unit 13 that controls the operating point ofthe optical modulating unit 11 is entered.

The operating point controlling unit 13 can keep the operating point ofthe optical modulating unit 11 in the optimum state when receiving apart of the optical signal that is exit from the optical modulating unit11.

On the other hand, the optical detecting unit 15 detects the intensityof the second branched input light that has been branched off by theoptical branching unit 10, and outputs a signal in accordance with thedetected light intensity. For example, the optical detecting unit 15outputs a signal when the light intensity is a predetermined value orless. Alternatively, the optical detecting unit 15 outputs a signal whenthe light intensity is zero. So that the operating point controllingunit 13 can keep the operating point stable, the signal that isgenerated in accordance with the light intensity is inputted to thecontrolling unit 14 that controls the operation of the operating pointcontrolling unit 13.

When receiving a signal from the optical detecting unit 15, thecontrolling unit 14 controls stopping the operation of the operatingpoint controlling unit 13. Alternatively, the controlling unit 14controls the operating point controlling unit 13 so that it keeps theoperating point in a limited range.

In this manner, the optical detecting unit 15 can detect whether theintensity of input light is a predetermined value or less. Therefore,when the intensity of the input light is the predetermined value orless, the controlling unit 14 can control, in accordance with the outputof the optical detecting unit 15, the operating point controlling unit13 so that it can keep the operating point stable. As a result, in theoptical communication apparatus having the above configuration, theoperating point can be kept stable even when input light is temporarilynon-existent.

Naturally, the optical detecting unit 15 does not output any signalswhen the intensity of input light is larger than the predetermined valueso as to use the optical modulating unit 11. Therefore, the operatingpoint controlling unit 13 controls the operating point to the optimumvalue based only on the output of the optical modulating unit 11 that isentered via the optical branching unit 12.

Next, the optical communication apparatus according to the secondembodiment will be explained based on the accompanying figure.

As shown in FIG. 2, the optical communication apparatus is composed ofan optical modulating unit 11, an optical branching unit 21, anoperating point controlling unit 13, a controlling unit 14, and anoptical detecting unit 23.

Light entered to an input port is modulated by the optical modulatingunit 11. The modulated optical signal is branched by the opticalbranching unit 21 that branches light into three.

The first optical signal that has been branched off by the opticalbranching unit 21 is exit to an output port. On the other hand, thesecond optical signal that has been branched off by the opticalbranching unit 21 is entered to the operating point controlling unit 13.

On the other hand, the optical detecting unit 23 detects the lightintensity of the third optical signal branched off by the opticalbranching unit 21, and outputs a signal in accordance with the detectedlight intensity. For example, the optical detecting unit 23 outputs asignal when the light intensity of the modulated optical signal is apredetermined value or less. Alternatively, the optical detecting unit23 outputs a signal when the light intensity is zero. The signal that isgenerated in accordance with the light intensity is inputted to thecontrolling unit 14.

When the intensity of input light is a predetermined value or less, theintensity of a modulated optical signal that is exit from the opticalmodulating unit 11 is also a predetermined value or less. Because ofthis, whether the intensity of the input light is the predeterminedvalue or less can be detected by detecting the light intensity of themodulated optical signal with the optical detecting unit 23. Therefore,the controlling unit 14 can control, in accordance with the output ofthe optical detecting unit 23, the operating point controlling unit 13so that it can keep the operating point stable. As a result, in theoptical communication apparatus having the above configuration, theoperating point can be kept stable even when input light is temporarilynon-existent.

Naturally, the optical detecting unit 15 does not output any signalswhen the intensity of input light is larger than the predetermined valueso as to use the optical modulating unit 11. Therefore, the operatingpoint controlling unit 13 controls the operating point to the optimumvalue only based on the output of the optical modulating unit 11 that isentered via the optical branching unit 21.

Next, the optical communication apparatus according to the thirdembodiment will be explained based on the accompanying figure.

As shown in FIG. 3, the optical communication apparatus is composed ofan optical modulating unit 11, an optical branching unit 12, anoperating point controlling unit 13, a controlling unit 25, and amodulation signal detecting unit 26.

Light entered to an input port is modulated by the optical modulatingunit 11. A modulated optical signal is branched by the optical branchingunit 12.

The first optical signal branched off by the optical branching unit 12is exit to an output port. On the other hand, the second optical signalbranched off by the optical branching unit 12 is entered to theoperating point controlling unit 13.

A modulation signal to be transmitted is inputted to not only theoptical modulating unit 11 but also the modulation signal detecting unit26. The modulation signal detecting unit 26 detects the intensity of themodulation signal and outputs a signal in accordance with the detectedsignal intensity. For example, the modulation signal detecting unit 26outputs a signal when the signal intensity becomes a predetermined valueor less. Alternatively, the modulation signal detecting unit 26 outputsa signal when the signal intensity becomes zero. The signal that isgenerated in accordance with the signal intensity is inputted to thecontrolling unit 25.

When receiving a signal from the modulation signal detecting unit 26,the controlling unit 25 stops the operation of the operating pointcontrolling unit 13. Alternatively, the controlling unit 25 controls theoperating point controlling unit 13 so that it keeps the operating pointin a limited range.

In this manner, the modulation signal detecting unit 26 can detectwhether the intensity of a modulation signal is a predetermined value orless. Therefore, when the intensity of the modulation signal is thepredetermined value or less, the controlling unit 25 can control, inaccordance with the output of the modulation signal detecting unit 26,the operating point controlling unit 13 so that it keeps the operatingpoint stable. As a result, in the optical communication apparatus havingthe above configuration, the operating point can be kept stable evenwhen modulation signal is temporarily non-existent.

Naturally, the modulation signal detecting unit 26 does not output anysignals when there is information to send and the intensity of amodulation signal to be transmitted is larger than the predeterminedvalue. Therefore, the operating point controlling unit 13 controls theoperating point to the optimum value only based on the output of theoptical modulating unit 11 that is entered via the optical branchingunit 12.

Next, the optical communication apparatus according to the fourthembodiment will be explained based on the accompanying figure.

In FIG. 4, this optical communication apparatus is composed of anoptical branching unit 10, an optical modulating unit 11, an opticalattenuating unit 31, an attenuation amount controlling unit 32, and anoptical detecting unit 33.

FIG. 4 shows a configuration in which first branched input light that isexit from the optical branching unit 10 is exit to an output port viathe optical attenuating unit 31 and the optical modulating unit 11.

On the other hand, as shown with broken lines in the same figure, theoptical communication apparatus can be configured as the output light isexit to an output port via the optical modulating unit 11 and theoptical attenuating unit 13.

Light entered to an input port is branched by the optical branching unit10. The first branched input light that has been branched off by theoptical branching unit 10 is entered, via the optical attenuating unit31, to the optical modulating unit 11, where it is modulated. Amodulated optical signal that is exit from the optical modulating unit11 is exit to the output port.

The optical attenuating unit 31 trajects or attenuates it topredetermined light intensity (including zero). Alternatively, theoptical attenuating unit 31 is a single input/plural output opticalswitch. When the optical attenuating unit 31 is such an optical switch,one output terminal is connected to the optical modulating unit 11 andthe other output terminal(s) are not connected to anything.

On the other hand, the optical detecting unit 33 detects the intensityof the second branched input light that has been branched off by theoptical branching unit 10, and outputs a signal in accordance with thedetected light intensity. For example, the optical detecting unit 33outputs a signal when the light intensity is a predetermined value orless. Or, the optical detecting unit 33 outputs a signal when the lightintensity is zero. The signal that is generated in accordance with thelight intensity is inputted to the attenuation amount controlling unit32.

The attenuation amount controlling unit 32 controls the opticalattenuating unit 31. That is, in accordance with a signal that isoutputted from the optical detecting unit 33, the attenuation amountcontrolling unit 32 controls the optical attenuating unit 31 so that itattenuates the input light to the predetermined intensity.Alternatively, where the optical attenuating unit 31 is an opticalswitch, in accordance with a signal that is outputted from the opticaldetecting unit 33, the attenuation amount controlling unit 32 switchesthe output of inputted light to an output terminal to which nothing isconnected.

In this manner, the optical detecting unit 33 can detect whether theintensity of input light is a predetermined value or less. Therefore,when the intensity of the input light is the predetermined value orless, the attenuation amount controlling unit 32 can output inputtedlight to the optical modulating unit 11 attenuating it to predeterminedlight intensity by controlling the optical attenuating unit 31 inaccordance with the output of the optical detecting unit 33.Alternatively, the attenuation amount controlling unit 32 can output theinput light to a terminal that is not connected to the opticalmodulating unit 11. As a result, in the optical communication apparatushaving the above configuration, ASE is not exit to the output port whenno input light exists.

Naturally, the optical detecting unit 33 does not output any signalswhen the intensity of input light is larger than the predetermined valueso as to use the optical modulating unit 11. At this time, theattenuation amount controlling unit 32 controls the optical attenuatingunit 31 so as to traject the input light or switch to the terminal thatis connected to the optical modulating unit 11.

Next, the optical communication apparatus according to the fifthembodiment will be explained based on the accompanying figure.

In FIG. 5, this optical communication apparatus is composed of anoptical branching unit 10, an optical modulating unit 11, an opticaldetecting unit 33, and a modulation controlling unit 35.

Light entered to an input port is branched by the optical branching unit10. The first branched input light that has been branched off by theoptical branching unit 10 is modulated by the optical modulating unit11, and the modulated optical signal is exit to an output port.

On the other hand, the optical detecting unit 33 detects the intensityof the second branched input light that has been branched off by theoptical branching unit 10, and outputs a signal in accordance with thedetected light intensity. The signal that is generated in accordancewith the light intensity is inputted to the modulation controlling unit35.

The modulation controlling unit 35 controls the optical modulating unit11. That is, in accordance with the signal that is outputted from theoptical detecting unit 33, the modulation controlling unit 35 controlsthe optical modulating unit 11 so that it attenuates the input light tothe predetermined intensity. For example, the modulation controllingunit 35 can prevent the optical modulating unit 11 from producing anyoutput by not supplying any energy to the optical modulating unit 11.Alternatively, where the optical modulating unit 11 is an MZ modulator,this can be done by shifting the phases of branched input light beamstransmitting through two respective optical waveguides in the MZmodulator by forming a phase difference of 180°. Alternatively, wherethe optical modulating unit 11 utilizes the acousto-optical effect, thiscan be done by applying to it an RF signal for selecting a wavelengthother than the wavelength of the input light.

In this manner, the optical detecting unit 33 can detect whether theintensity of input light is a predetermined value or less. Therefore,when the intensity of the input light is the predetermined value orless, the modulation controlling unit 35 can prevent the opticalmodulating unit 11 from producing any output by controlling it inaccordance with the output of the optical detecting unit 33. As aresult, in the optical communication apparatus having the aboveconfiguration, neither ASE nor input light that is not modulated with amodulation signal is exit to the output port even when input lightexists but no modulation signal exists.

Naturally, the optical detecting unit 33 does not output any signalswhen the intensity of input light is larger than the predetermined valueso as to use the optical modulating unit 11. At this time, the opticalmodulating unit 11 operates normally as a modulating unit because itdoes not receive a signal from the modulation controlling unit 35.

Next, the optical communication apparatus according to the sixthembodiment will be explained based on the accompanying figure.

In FIG. 6, this optical communication apparatus is composed of anoptical attenuating unit 31, an optical modulating unit 11, anattenuation amount controlling unit 41, and a modulation signaldetecting unit 42.

FIG. 6 shows a configuration in which light entered to an input port isexit to an output port via the optical attenuating unit 31 and theoptical modulating unit 11.

On the other hand, as shown with broken lines in the same figure, theoptical communication apparatus can be configured as the output light isexit to an output port via the optical modulating unit 11 and theoptical attenuating unit 13.

Light entered to the input port is entered, via the optical attenuatingunit 31, to the optical modulating unit 11, where it is modulated. Amodulated optical signal that is exit from the optical modulating unit11 is exit to the output port.

A modulation signal to be transmitted is inputted to not only theoptical modulating unit 11 but also the modulation signal detecting unit42. The modulation signal detecting unit 42 detects the intensity of themodulation signal and outputs a signal in accordance with the detectedsignal intensity. For example, the modulation signal detecting unit 42outputs a signal when the signal intensity becomes a predetermined valueor less. Alternatively, the modulation signal detecting unit 42 outputsa signal when the signal intensity becomes zero. The signal that isgenerated in accordance with the signal intensity is inputted to theattenuation amount controlling unit 41.

The attenuation amount controlling unit 41 controls the opticalattenuating unit 31. That is, in accordance with a signal that isoutputted from the modulation signal detecting unit 42, the attenuationamount controlling unit 41 controls the optical attenuating unit 31 sothat it attenuates the input light to predetermined light intensity.Alternatively, where the optical attenuating unit 31 is an opticalswitch, in accordance with a signal that is outputted from themodulation signal detecting unit 33, the attenuation amount controllingunit 41 switches the output of the inputted light to an output terminalto which nothing is connected.

In this manner, the modulation signal detecting unit 42 can detectwhether the intensity of a modulation signal is a predetermined value orless. Therefore, when the intensity of the modulation signal is thepredetermined value or less, the attenuation amount controlling unit 41can exit inputted light to the optical modulating unit 11 attenuating itto predetermined light intensity by controlling the optical attenuatingunit 31 in accordance with the output of the modulation signal detectingunit 42. Alternatively, the attenuation amount controlling unit 41 canexit inputted light to a terminal that is not connected to the opticalmodulating unit 11. As a result, in the optical communication apparatushaving the above configuration, ASE is not exit to the output port whenno input light exists. Further, neither ASE nor input light that is notmodulated with a modulation signal is exit to the output port even wheninput light exists but no modulation signal exists.

Naturally, the modulation signal detecting unit 42 does not output anysignals when there is information to send and the intensity of amodulation signal to be transmitted is larger than the predeterminedvalue. At this time, the attenuation amount controlling unit 41 controlsthe optical attenuating unit 31 so that it trajects the input light orswitches to the terminal that is connected to the optical modulatingunit 11.

The optical communication apparatus according to the seventh embodimentwill be explained based on the accompanying figure.

In FIG. 7, this optical communication apparatus is composed of anoptical modulating unit 11, a modulation signal detecting unit 42, and amodulation controlling unit 45.

Light entered to an input port is modulated by the optical modulatingunit 11. A modulated optical signal is exit to an output port.

A modulation signal to be transmitted is inputted to not only theoptical modulating unit 11 but also to the modulation signal detectingunit 42. The modulation signal detecting unit 42 outputs a signal inaccordance with the intensity of the modulation signal. The signal thatis outputted from the modulation signal detecting unit 42 is inputted tothe modulation controlling unit 45.

The modulation controlling unit 45 controls the optical modulating unit11. That is, in accordance with a signal that is outputted from themodulation signal detecting unit 42, the modulation controlling unit 45controls the optical modulating unit 11 so that it attenuates the inputlight to the predetermined light intensity. For example, the modulationcontrolling unit 45 can prevent the optical modulating unit 11 fromproducing any outputs by not supplying any energy to it. Alternatively,where the optical modulating unit 11 is an MZ modulator, this can bedone by shifting the phases of branched input light beams transmittingthrough two respective optical waveguides in the MZ modulator by forminga phase difference of 180°. As a further alternative, where the opticalmodulating unit 11 utilizes the acousto-optical effect, this can be doneby applying to it an RF signal for selecting a wavelength other than thewavelength of the input light.

In this manner, the modulation signal detecting unit 42 can detectwhether the intensity of a modulation signal is a predetermined value orless. Therefore, when the intensity of the modulation signal is thepredetermined value or less, the modulation controlling unit 45 canprevent the optical modulating unit 11 from producing any outputs bycontrolling it. As a result, in the optical communication apparatushaving the above configuration, neither ASE nor input light that is notmodulated with a modulation signal is exit to the output port even wheninput light exists but no modulation signal exists.

Naturally, the modulation signal detecting unit 42 does not output anysignals when there is information to send and the intensity of amodulation signal to be transmitted is larger than the predeterminedvalue. At this time, the optical modulating unit 11 operates normally asa modulating unit because it does not receive a signal from themodulation controlling unit 45.

Next, the optical communication apparatus according to the eighthembodiment will be explained based on the accompanying figure.

As shown in FIG. 8, this optical communication apparatus is composed ofoptical branching unit 10 and 12, an optical attenuating unit 50, anoptical modulating unit 11, an operating point controlling unit 13, acontrolling unit 14, and an optical detecting unit 15.

Light entered to an input port is branched by the optical branching unit10. First branched input light is entered to the optical attenuatingunit 50. Output light of the optical attenuating unit 50 is modulated bythe optical modulating unit 11. A modulated optical signal is branchedby an optical branching unit 12. The first optical signal branched offby the optical branching unit 12 is exit to an output port. On the otherhand, the second optical signal branched off by the optical branchingunit 12 is entered to the operating point controlling unit 13.

The optical attenuating unit 50 trajects received input light orattenuates it to predetermined light intensity (including zero) inaccordance with the intensity of the input light. Alternatively, theoptical attenuating unit 50 is a single input/plural output opticalswitch. Where the optical attenuating unit 50 is such an optical switch,one output terminal is connected to the optical modulating unit 11 andthe other output terminal(s) are not connected to anything.

On the other hand, the optical detecting unit 15 detects the intensityof second branched input light that has been branched off by the opticalbranching unit 10, and outputs a signal in accordance with the detectedlight intensity. The signal that is generated in accordance with thelight intensity is inputted to the controlling unit 14.

The optical communication apparatus having the above configuration notonly operates in the same manner as the optical communication apparatusaccording to the first embodiment of the invention but also does notexit ASE to the output port when no input light exists.

That is, the optical attenuating unit 50 judges whether the intensity ofthe light received is a predetermined value or less and attenuates thelight received in accordance with the judgment result. Therefore, whenthe intensity of the light received is the predetermined value or less,the optical attenuating unit 50 attenuates it to predetermined intensityand outputs resulting light. Alternatively, where the opticalattenuating unit 50 is an optical switch, when the intensity of thelight received is the predetermined value or less, the opticalattenuating unit 50 switches to outputting the light received to anoutput terminal to which nothing is connected. Therefore, the opticalcommunication apparatus having the above configuration does not exit ASEto the output port when no input light exists.

Naturally, the optical attenuating unit 50 trajects input light andoutputs it when the intensity of the input light is larger than thepredetermined value so as to use the optical modulating unit 11.Alternatively, where the optical attenuating unit 50 is an opticalswitch, it switches to outputting the light received to the terminalthat is connected to the optical modulating unit 11.

Note that, the optical attenuating unit 50 placed at the input of theoptical modulating unit 11 in this optical communication apparatus asshown in FIG. 8 can also be placed at the output of the same.

Next, the optical communication apparatus according to the ninthembodiment will be explained based on the accompanying figure.

As shown in FIG. 9, this optical communication apparatus is composed ofoptical branching unit 10 and 12, an optical modulating unit 55, anoperating point controlling unit 13, a controlling unit 14, and anoptical detecting unit 15.

Light entered to an input port is branched by the optical branching unit10. First branched input light is modulated by the optical modulatingunit 55 in accordance with a modulation signal to be transmitted. Themodulated optical signal that is exit from the optical modulating unit55 is branched by the optical branching unit 12 that branches light intotwo beams.

The optical modulating unit 55 controls whether to output a modulatedoptical signal, in accordance with the intensity of the modulationsignal or the intensity of the light received.

The first optical signal branched off by the optical branching unit 12is exit to an output port. On the other hand, the second optical signalbranched off by the optical branching unit 12 is entered to theoperating point controlling unit 13.

On the other hand, the optical detecting unit 15 detects the intensityof second branched input light that has been branched off by the opticalbranching unit 10, and outputs a signal in accordance with the detectedlight intensity. This signal is inputted to the controlling unit 14.

The optical communication apparatus having the above configuration notonly operates in the same manner as the optical communication apparatusaccording to the first embodiment of the invention but also does notexit ASE to the output port when no input light exists.

The optical modulating unit 55 judges whether the intensity of the lightreceived is a predetermined value or less or whether the intensity ofthe modulation signal is a predetermined value or less. As a result, theoptical modulating unit 55 produces no output when the intensity of thelight received is the predetermined value or less or the intensity ofthe modulation signal is the predetermined value or less. For example,it is possible to prevent the optical modulating unit 55 from producingany outputs by not supplying energy to it. Alternatively, where theoptical modulating unit 11 is an MZ modulator, this can be done byshifting the phases of branched light beams transmitting through tworespective optical waveguides in the MZ modulator by forming a phasedifference of 180°. Alternatively, where the optical modulating unit 11utilizes the acousto-optical effect, this can be done by applying to itan RF signal for selecting a wavelength other than the wavelength of theinput light. Therefore, the optical communication apparatus having theabove configuration does not exit ASE nor input light that is notmodulated with a modulation signal even when input light exists but nomodulation signal exists.

Naturally, the optical modulating unit 55 operates normally as amodulating unit when the intensity of the input light is larger than thepredetermined value or when the intensity of the modulation signal islarger than the predetermined value so as to use the optical modulatingunit 55.

Note that, the controlling unit 14 controlled by the opticalcommunication apparatus according to the input light of the opticalmodulating unit 55 as shown in FIG. 9 can also be controlled accordingto the output light of the optical modulating unit 55 or a modulationsignal.

Next, the optical communication apparatus according to the tenthembodiment will be explained based on the accompanying figure.

In FIG. 10, this optical communication apparatus is composed of opticalbranching unit 10 and 12, an optical modulating unit 11, an operatingpoint controlling unit 13, a controlling unit 14, an optical detectingunit 15, an optical attenuating unit 31, an attenuation amountcontrolling unit 41, and a modulation signal detecting unit 42.

Light entered to an input port is branched by the optical branching unit10. First branched input light is entered, via the optical attenuatingunit 31, to the optical modulating unit 11, where it is modulated. Amodulated optical signal is branched by the optical branching unit 12.

The first optical signal branched off by the optical branching unit 12is exit to an output port. On the other hand, the second optical signalbranched off by the optical branching unit 12 is entered to theoperating point controlling unit 13.

On the other hand, the optical detecting unit 15 detects the intensityof second branched input light that has been branched off by the opticalbranching unit 10, and outputs a signal in accordance with the detectedlight intensity. The signal that is generated in accordance with thelight intensity is inputted to the controlling unit 14.

A modulation signal to be transmitted is inputted to not only theoptical modulating unit 11 but also the modulation signal detecting unit42. The modulation signal detecting unit 42 detects the intensity of themodulation signal and outputs a signal in accordance with the detectedsignal intensity, which is inputted to the attenuation amountcontrolling unit 41.

In this manner, the optical detecting unit 15 can detect whether theintensity of input light is a predetermined value or less. Therefore,when the intensity of the input light is the predetermined value orless, the controlling unit 14 can control, in accordance with the outputof the optical detecting unit 15, the operating point controlling unit13 so that it keeps the operating point stable. As a result, in theoptical communication apparatus having the above configuration, theoperating point can be kept stable even when input light is temporarilynon-existent.

Naturally, the optical detecting unit 15 does not output any signalswhen the intensity of input light is larger than the predetermined valueso as to use the optical modulating unit 11. Therefore, the operatingpoint controlling unit 13 controls the operating point to the optimumvalue only based on the output of the optical modulating unit 11 that isentered via the optical branching unit 12.

Further, the modulation signal detecting unit 42 can detect whether theintensity of a modulation signal is a predetermined value or less.Therefore, when the intensity of the modulation signal is thepredetermined value or less, the attenuation amount controlling unit 41can exit inputted light to the optical modulating unit 11 attenuating itto predetermined light intensity by controlling the optical attenuatingunit 31 in accordance with the output of the modulation signal detectingunit 42. Alternatively, the attenuation amount controlling unit 41 canexit the input light to a terminal that is not connected to the opticalmodulating unit 11. As a result, in the optical communication apparatushaving the above configuration, ASE is not exit to the output port whenno input light exists. Further, neither ASE nor input light that is notmodulated with a modulation signal is exit to the output port even wheninput light exists but no modulation signal exists.

Naturally, the modulation signal detecting unit 42 does not output anysignals when there is information to send and the intensity of amodulation signal to be transmitted is larger than the predeterminedvalue. At this time, the attenuation amount controlling unit 41 controlsthe optical attenuating unit 31 so that it trajects input light orcauses the optical attenuating unit 11 to switch to supplying the lightreceived to the terminal that is connected to the optical modulatingunit 11.

Note that, in this optical communication apparatus, the controlling unit14 can be controlled according to the detection done by the opticaldetecting unit 15 on light output from the optical modulating unit 11together with having the optical attenuating unit 31 placed at theoutput of optical modulating unit 11, as shown in broken lines, insteadof having the optical detecting unit 15 detect input light together withhaving the optical attenuating unit 31 placed at the input of opticalmodulating unit 11, as shown in FIG. 10.

Next, the optical communication apparatus according to the eleventhembodiment will be explained based on the accompanying figure.

In FIG. 11, this optical communication apparatus is composed of opticalbranching unit 10 and 12, an optical modulating unit 11, an operatingpoint controlling unit 13, a controlling unit 14, an optical detectingunit 15, a modulation signal detecting unit 42, and a modulationcontrolling unit 45.

Light entered to an input port is branched by the optical branching unit10. First branched input light is entered to the optical modulating unit11, where it is modulated. A modulated optical signal is branched by theoptical branching unit 12.

The first optical signal branched off by the optical branching unit 12is exit to an output port. On the other hand, the second optical signalbranched off by the optical branching unit 12 is entered to theoperating point controlling unit 13.

On the other hand, the optical detecting unit 15 detects the intensityof second branched input light that has been branched off by the opticalbranching unit 10, and outputs a signal in accordance with the detectedlight intensity, which is inputted to the controlling unit 14.

A modulation signal to be transmitted is inputted to not only theoptical modulating unit 11 but also the modulation signal detecting unit42. The modulation signal detecting unit 42 outputs a signal inaccordance with the intensity of the modulation signal, and the signalthat is outputted is inputted to the modulation controlling unit 45.

In this manner, the optical detecting unit 15 can detect whether theintensity of input light is a predetermined value or less. Therefore,when the intensity of the input light is the predetermined value orless, the controlling unit 14 can control, in accordance with the outputof the optical detecting unit 15, the operating point controlling unit13 so that it keeps the operating point stable. As a result, in theoptical communication apparatus having the above configuration, theoperating point can be kept stable even when input light is temporarilynon-existent.

Naturally, the optical detecting unit 15 does not output any signalswhen the intensity of input light is larger than the predetermined valueso as to use the optical modulating unit 11. Therefore, the operatingpoint controlling unit 13 controls the operating point to the optimumvalue only based on the output of the optical modulating unit 11 that isentered via the optical branching unit 12.

Further, the modulation signal detecting unit 42 can detect whether theintensity of a modulation signal is a predetermined value or less.Therefore, when the intensity of the modulation signal is thepredetermined value or less, the modulation controlling unit 45 canprevent the optical modulating unit 11 from producing any output bycontrolling the optical modulating unit 11. As a result, in the opticalcommunication apparatus having the above configuration, neither ASE norinput light that is not modulated with a modulation signal is exit tothe output port even when input light exists but no modulation signalexists.

Naturally, the modulation signal detecting unit 42 does not output anysignals when there is information to send and the intensity of amodulation signal to be transmitted is larger than the predeterminedvalue. At this time, the optical modulating unit 11 operates normally asa modulating unit because it does not receive a signal from themodulation controlling unit 45.

Note that, the controlling unit 14 controlled by the opticalcommunication apparatus according to the input light of the opticalmodulating unit 11 as shown in FIG. 11 can also be controlled accordingto the output light of the optical modulating unit 11 or a modulationsignal as shown with broken lines.

Next, the optical communication apparatus according to the twelfthembodiment will be explained based on the accompanying figure.

In FIG. 12, this optical communication apparatus is composed of opticalbranching unit 10 and 12, an optical modulating unit 11, an operatingpoint controlling unit 13, a controlling unit 14, an optical detectingunit 15, an optical attenuating unit 31, an attenuation amountcontrolling unit 61, and a modulation signal detecting unit 42.

Light entered to an input port is branched by the optical branching unit10. First branched input light is entered, via the optical attenuatingunit 31, to the optical modulating unit 11, where it is modulated. Amodulated optical signal is branched by the optical branching unit 12.

The first optical signal branched off by the optical branching unit 12is exit to an output port. On the other hand, the second optical signalbranched off by the optical branching unit 12 is entered to theoperating point controlling unit 13.

On the other hand, the optical detecting unit 15 detects the intensityof second branched input light that has been branched off by the opticalbranching unit 10, and outputs a signal in accordance with the detectedlight intensity, which is inputted to the controlling unit 14 and theattenuation amount controlling unit 61.

A modulation signal to be transmitted is inputted to not only theoptical modulating unit 11 but also the modulation signal detecting unit42, where the intensity of the modulation signal is detected and asignal in accordance with the detected signal intensity is outputted.The output signal in accordance with the signal intensity is inputted tothe attenuation amount controlling unit 61.

The attenuation amount controlling unit 61 controls the opticalattenuating unit 31. That is, the attenuation amount controlling unit 61calculates the AND of the signal of the optical detecting unit 15 andthe signal of the modulation signal detecting unit 42, and controls theoptical attenuating unit 31 so that it attenuates the light received topredetermined light intensity. Alternatively, where the opticalattenuating unit 31 is an optical switch, in accordance with the signalof the optical detecting unit 15, the attenuation amount controllingunit 61 switches the output of the inputted light to an output terminalto which nothing is connected.

In this manner, the optical detecting unit 15 can detect whether theintensity of input light is a predetermined value or less. Therefore,when the intensity of the input light is the predetermined value orless, the controlling unit 14 can control, in accordance with the outputof the optical detecting unit 15, the operating point controlling unit13 so that it can keep the operating point stable. As a result, in theoptical communication apparatus having the above configuration, theoperating point can be kept stable even when input light is temporarilynon-existent.

Naturally, the optical detecting unit 15 does not output any signalswhen the intensity of input light is larger than the predetermined valueso as to use the optical modulating unit 11. Therefore, the operatingpoint controlling unit 13 controls the operating point to the optimumvalue only based on the output of the optical modulating unit 11 that isentered via the optical branching unit 12.

Further, the modulation signal detecting unit 42 can detect whether theintensity of a modulation signal is a predetermined value or less. Theattenuation amount controlling unit 61 also receives a signal from theoptical detecting unit 15 and calculates the AND of this signal and asignal of the modulation signal detecting unit 42. Therefore, when theintensity of the input light is the predetermined value or less or whenthe intensity of the modulation signal is the predetermined value orless, the attenuation amount controlling unit 61 can output inputtedlight to the optical modulating unit 11 after being attenuated topredetermined light intensity by controlling the optical attenuatingunit 31. Alternatively, the attenuation amount controlling unit 61 canhave the optical attenuating unit 31 exit the light received to aterminal that is not connected to the optical modulating unit 11. As aresult, in the optical communication apparatus having the aboveconfiguration, ASE is not exit to the output port when no input lightexists. Further, neither ASE nor input light that is not modulated witha modulation signal is exit to the output port even when input lightexists but no modulation signal exists.

Naturally, when there is information to send, the intensity of amodulation signal to be transmitted is larger than the predeterminedvalue, and the intensity of input light is larger than the predeterminedvalue, the attenuation amount controlling unit 61 controls the opticalattenuating unit 31 so that it trajects the input light or switches theoutput of the inputted light to the terminal that is connected to theoptical modulating unit 11.

Note that, in this optical communication apparatus, the controlling unit14 and attenuation amount controlling unit 61 can be controlledaccording to the detection done by the optical detecting unit 15 onlight output from the optical modulating unit 11 together with havingthe optical attenuating unit 31 placed at the output of opticalmodulating unit 11, as shown in broken lines, instead of having theoptical detecting unit 15 detect input light together with having theoptical attenuating unit 31 placed at the input of optical modulatingunit 11, as shown in FIG. 12.

Next, the optical communication apparatus according to the thirteenthembodiment will be explained based on the accompanying figure.

In FIG. 13, this optical communication apparatus is composed of opticalbranching unit 10 and 12, an optical modulating unit 11, an operatingpoint controlling unit 13, a controlling unit 14, an optical detectingunit 15, a modulation signal detecting unit 42, and a modulationcontrolling unit 65.

Light entered to an input port is branched by the optical branching unit10. First branched input light is entered to the optical modulating unit11, where it is modulated. A modulated optical signal is branched by theoptical branching unit 12.

The first optical signal branched off by the optical branching unit 12is outputted to an output port. On the other hand, the second opticalsignal branched off by the optical branching unit 12 is entered to theoperating point controlling unit 13.

On the other hand, the optical detecting unit 15 detects the intensityof second branched input light that has been branched off by the opticalbranching unit 10, and outputs a signal in accordance with the detectedlight intensity. The inputted to the controlling unit 14 and themodulation controlling unit 65.

A modulation signal to be transmitted is inputted to not only theoptical modulating unit 11 but also the modulation signal detecting unit42. The modulation signal detecting unit 42 outputs a signal inaccordance with the intensity of the modulation signal, which isinputted to the modulation controlling unit 65.

The modulation controlling unit 65 controls the optical modulating unit11. That is, the modulation controlling unit 65 calculates the AND ofthe signal from the optical detecting unit 15 and the signal from themodulation signal detecting unit 42, and controls the optical modulatingunit 11 so that it attenuates the light received to a predeterminedlight intensity. For example, the modulation controlling unit 65 canprevent the optical modulating unit 11 from producing any outputs by notsupplying energy to it. Alternatively, where the optical modulating unit11 is an MZ modulator, the modulation controlling unit 65 can preventthe optical modulating unit 11 from producing any outputs by shiftingthe phases of branched input light beams transmitting through tworespective optical waveguides in the MZ modulator so as to form a phasedifference of 180°. As a further alternative, where the opticalmodulating unit 11 utilizes the acousto-optical effect, the modulationcontrolling unit 65 can prevent the optical modulating unit 11 fromproducing any outputs by applying to it an RF signal for selecting awavelength other than the wavelength of the input light.

In this manner, the optical detecting unit 15 can detect whether theintensity of input light is a predetermined value or less. Therefore,when the intensity of the input light is the predetermined value orless, the controlling unit 14 can control, in accordance with the outputof the optical detecting unit 15, the operating point controlling unit13 so that it keeps the operating point stable. As a result, in theoptical communication apparatus having the above configuration, theoperating point can be kept stable even when input light is temporarilynon-existent.

Naturally, the optical detecting unit 15 does not output any signalswhen the intensity of input light is larger than the predetermined valueso as to use the optical modulating unit 11. Therefore, the operatingpoint controlling unit 13 controls the operating point to the optimumvalue based only on the output of the optical modulating unit 11 that isentered via the optical branching unit 12.

Further, the modulation signal detecting unit 42 can detect whether theintensity of a modulation signal is a predetermined value or less. Themodulation controlling unit 65 also receives a signal from the opticaldetecting unit 15 and calculates the AND of this signal and a signal ofthe modulation signal detecting unit 42. Therefore, when the intensityof the input light is the predetermined value or less or when theintensity of the modulation signal is the predetermined value or less,the modulation controlling unit 65 can prevent the optical modulatingunit 11 from producing outputs by controlling the optical modulatingunit 11. As a result, in the optical communication apparatus having theabove configuration, neither ASE nor input light that is not modulatedwith a modulation signal is outputted to the output port even when inputlight exists but no modulation signal exists.

Naturally, neither the modulation signal detecting unit 42 nor theoptical detecting unit 15 output a signal to the modulation controllingunit 65 when there is information to send, the intensity of a modulationsignal to be transmitted is larger than the predetermined value, and theintensity of input light is larger than the predetermined value. As aresult, the optical modulating unit 11 operates normally as a modulatingunit because it does not receive a signal from the modulationcontrolling unit 65.

Note that, the controlling unit 14 controlled by the opticalcommunication apparatus according to the input light of the opticalmodulating unit 11 as shown in FIG. 13 can also be controlled accordingto the output light of the optical modulating unit 11 or a modulationsignal as shown with broken lines.

Next, the optical communication apparatus according to the fourteenthembodiment will be explained based on the accompanying figure.

As shown in FIG. 14, this optical add/drop apparatus is composed of anadd/drop unit 70, an optical wavelength branching unit 73, and anoptical adding unit 75.

Actually, the optical add/drop apparatus is provided with opticalwavelength branching unit 73 and optical adding unit 75 in the numberequivalent to the number of wavelengths of light beams to be added ordropped. However, each optical wavelength branching unit 73 is the samein configuration and different only in the wavelength of light on whichto perform a receiving operation, and each optical adding unit 75 is thesame in configuration and different only in the wavelength of light tobe added. Therefore, in FIG. 14, only one of the plurality of opticalwavelength branching unit 73 and only one of the plurality of opticaladding unit 75 are shown by solid lines and the other unit are shownwith broken lines.

The add/drop unit 70 is connected to an optical transmission line fortransmitting through a wavelength-division multiplexed optical signal,and adds and drops optical signals of at least one wavelength to andfrom an optical signal transmitting through the optical transmissionline. When dropping, optical signals are dropped to optical wavelengthbranching unit 73 via an optical distributing unit 72 that distributesoptical signals in accordance with the number of optical wavelengthbranching unit 73. When adding, an optical multiplexing unit 74multiplexes adding optical signals coming from respective optical addingunit 75 wavelength-division multiplexes the multiplexed addition lightwith an optical signal trajecting the optical add/drop apparatus andoutputted to the optical transmission line.

The optical wavelength branching unit 73 performs receiving operationson distributed optical signals of respective wavelengths. On the otherhand, the optical adding unit 75 generates addition light beams to beadded to an optical signal on the optical transmission line.

The configuration of each optical adding unit 75 will be describedbelow, which is composed of optical branching unit 10 and 12, an opticalmodulating unit 11, an operating point controlling unit 13, acontrolling unit 14, and an optical detecting unit 15.

Input light of a particular wavelength that is entered to an input portis branched by the optical branching unit 10. First branched input lightthat has been branched off by the optical branching unit 10 is modulatedby the optical modulating unit 11. A modulated optical signal isbranched by the optical branching unit 12. Each particular wavelength isdifferent among the respective optical adding unit 75.

The first optical signal branched off by the optical branching unit 12is exit to an output port and entered to the optical multiplexing unit74. On the other hand, the second optical signal branched off by theoptical branching unit 12 is entered to the operating point controllingunit 13 that controls the operating point of the optical modulating unit11.

On the other hand, the optical detecting unit 15 detects the intensityof second branched input light that has been branched off by the opticalbranching unit 10, and outputs a signal in accordance with the detectedlight intensity, which is inputted to the controlling unit 14.

In this manner, the optical detecting unit 15 can detect whether theintensity of input light is a predetermined value or less. Therefore,when the intensity of the input light is the predetermined value orless, the controlling unit 14 can control, in accordance with the outputof the optical detecting unit 15, the operating point controlling unit13 so that it can keep the operating point stable. As a result, in theoptical add/drop apparatus having the above configuration, the operatingpoint of the optical adding unit 75 can be kept stable even when theoptical adding unit 75 has no input light because no addition light isto be supplied to the add/drop unit 70.

Naturally, the optical detecting unit 15 does not output any signalswhen the intensity of input light is larger than the predetermined valueto generate addition light by the optical adding unit 75. Therefore, theoperating point controlling unit 13 controls the operating point to theoptimum value only based on the output of the optical modulating unit 11that is entered via the optical branching unit 12.

Note that, the optical communication apparatus according to the firstembodiment applied as optical adding unit 75 in the fourteenthembodiment can be replaced by the optical communication apparatuses inthe second through the thirteenth embodiments.

Next, the fifteenth embodiment will be explained.

First, the configuration of the fifteenth embodiment will be described.

In FIG. 15, this optical add/drop apparatus is composed of opticalamplifiers 101 and 103, an OADM 102, an 1×M optical coupler 104, Moptical wavelength branching circuits 105, an N×1 opticalmulti/demultiplexer 106, and N optical addition circuits 107 a.

Although this optical add/drop apparatus has the M optical wavelengthbranching circuits 105 and the N optical addition circuits 107 a, inFIG. 15 only one of the M optical wavelength branching circuits 105 andonly one of the N optical addition circuits 107 a are shown by solidlines and the other circuits are shown with broken lines because thecircuits of each group have the same configuration.

A wavelength-division multiplexed optical signal transmitting through anoptical transmission line enters the optical add/drop apparatus and isamplified by the amplifier 101 that amplifies an optical signal topredetermined light intensity. The amplified optical signal is enteredto the OADM 102 that adds or drops a wavelength-division multiplexedoptical signal. Signal light beams of predetermined wavelengths thathave been dropped by the OADM 102 are entered to the 1×M optical coupler104 that divides the optical signal(s) into as many optical wavelengthbranching circuits there are. The optical signal(s) distributed by the1×M optical coupler 104 are entered to and performed receivingoperations on optical signals of respective wavelengths in the opticalwavelength branching circuits 105. On the other hand, optical signals tobe added in the OADM 102 are generated by the optical addition circuits107 a, which are provided in N, that is, the number of optical signalsto be added in the OADM 102. The optical signals to be added and anoptical signal that has not been dropped in the OADM 102 arewavelength-division multiplexed with each other, and a resulting opticalsignal is amplified by the optical amplifier 103 and then outputted tothe optical transmission line.

Each optical addition circuit 107 a is composed of a laser diode bank(hereinafter abbreviated as “LD bank”) 110, optical amplifiers 111 and115, optical couplers 112 and 114, an MZ modulator 113, PDs 116 and 123,amplifiers 117 and 121, a comparator 118, a switch 119, a variable gainamplifier 120, a coupling capacitor 122, a buffer amplifier 124, amultiplier 125, an LPF 126, a differential amplifier 127, an inductor128, a capacitor 129, a resistor 130, and a low-frequency oscillator131.

The circuit composed of the variable gain amplifier 120, the amplifier121, the coupling capacitor 122, the PD 123, the buffer amplifier 124,the multiplier 125, the LPF 126, the differential amplifier 127, theinductor 128, the capacitor 129, the resistor 130, and the low-frequencyoscillator 131 is called an operating point control circuit.

In FIG. 15, the LD bank 110 can exit laser beams of a plurality ofwavelengths L1–L8 corresponding to the wavelengths forwavelength-division multiplexing. The wavelength of light to be exitedactually is selected in accordance with a detection signal that isgenerated by detecting available wavelengths of the optical transmissionline with a wavelength monitor (not shown in FIG. 15). For example, theLD bank 110 exits light of a wavelength L2, which is entered to theoptical amplifier 111. Amplified light is branched into two beams by theoptical coupler 112, and first branched light is entered to the MZmodulator 113.

A modulation signal and a low-frequency signal of a predeterminedfrequency f0 that is outputted from the low-frequency oscillator 131 areinputted to the variable gain amplifier 120. The variable gain amplifier120 amplitude-modulates and outputs the signal received. The outputsignal is inputted to one modulation-input terminal of the MZ modulator113 via the amplifier 121 which gains a predetermined signal level andthe coupling capacitor 122.

The resistor 130 and a bias T circuit that is composed of the inductor128 and the capacitor 129 are connected to the other modulation-inputterminal of the MZ modulator 113.

The MZ modulator 113 modulates the light of the wavelength L2 that isexit from the LD bank 110 into an optical signal with the signalsupplied from the drive circuit and outputs it.

Part of the output light of the MZ modulator 113 is branched off by theoptical coupler 114 and thereby taken out. The other part of the outputlight is amplified by the optical amplifier 115 and then entered to theabove-mentioned N×1 optical multi/demultiplexer 106. The branched partof the output light is detected by the PD 123. A detection signal isamplified by the buffer amplifier 124 that selectively amplifies afrequency component of f0, and is inputted to the multiplier 125. Thelow-frequency signal that is outputted from the low-frequency oscillator131 is also inputted to the multiplier 125. The multiplier 125 comparesthe phases of the input signal supplied from the buffer amplifier 124and the low-frequency signal supplied from the low-frequency oscillator131, and outputs a signal in accordance with a phase difference. Themultiplexer 125 detects the low-frequency signal of the predeterminedfrequency f0 that was superimposed by the variable gain amplifier 120.

The output signal of the multiplier 125 is inputted to one inputterminal of the differential amplifier 127 via the LPF 126 that allowspassage of a frequency component of the predetermined frequency f0 orless and the switch 119. The other input terminal of the differentialamplifier 127 is grounded. An output of the differential amplifier 127is inputted to the inductor 128 of the bias T circuit as an error signalto be used for moving the operating point, and the bias value isvariably controlled so as to correct the operating point.

On the other hand, second branched light that has been branched off bythe optical coupler 112 is entered to the PD 116, which outputs anelectrical signal that is in proportion to the average intensity of thesecond branched light. That is, the PD 116 detects the intensity of thelight that is exit from the LD bank 110.

The electrical signal that is outputted from the PD 116 is amplified bythe amplifier 117 and then compared with a reference voltage Vref by thecomparator 118. When the electrical signal is smaller than or equal tothe reference voltage Vref, the comparator 118 outputs a signal to theswitch 119 and controls the on/off of switch 119.

When receiving a signal from the comparator 118, the switch 119 isturned off to disconnect the LPF 126 from the differential amplifier127. During the period when no signals are received by the comparator118, the switch 119 is turned on to connect the LPF 126 with thedifferential amplifier 127.

Next, functions and advantageous effects of the fifteenth embodimentwill be described.

The optical add/drop apparatus having the above configuration can keepthe operating point stable even if the input light does not exist duringthe period when the wavelength of light exit from the LD bank 110 ischanged in the optical addition circuit 107 a, for example, during theperiod when laser light of wavelength L2 is switched to laser light ofwavelength L4.

This will be explained below in the case where the wavelength L2 isswitched to the wavelength L4.

At first, since a wavelength-division multiplexed signal transmittingthrough the optical transmission line has an available wavelength L2,the LD bank 110 exits light of the wavelength L2. The exit light ismodulated by the MZ modulator 113, added in the OADM102 as an additionlight via the N×1 optical multi/demultiplexer 106, and inputted to theoptical transmission line. The exit light is also entered to theoperating point control circuit, where it is used to control theoperating point of the MZ modulator 113. The exit light is alsophotoelectrically converted by the PD 116, and an output signal of thePD 116 is judged as to whether it is smaller than or equal to thereference voltage Vref by the comparator 118. That is, whether theintensity of the light of the wavelength L2 exit from the LD bank 110 isa predetermined value or less can be judged by the comparator 118 as towhether the electrical signal that is outputted from the PD 116 issmaller than or equal to the predetermined reference voltage Vref.

Since the light of the wavelength L2 exit from the LD bank 110 is usedas addition light, its light intensity is larger than the predeterminedvalue and hence the comparator 118 does not output any signals.Therefore, the switch 119 is kept on and LPF 126 is kept connected withdifferential amplifier 127. As a result, the operating point controlcircuit continues to operate normally.

Then, the available wavelength of a wavelength-division multiplexedsignal transmitting through the optical transmission line is changedfrom L2 to L4, whereupon the LD bank 110 stops exiting the light of thewavelength L2.

At this time, the level of the output signal of the PD 116 decreases toapproximately zero. Since the output signal is smaller than thereference voltage Vref, the comparator 118 sends a signal to the switch119. The switch 119 is turned off and the LPF 126 is disconnected fromthe differential amplifier 127. As a result, the operating point controlcircuit stops operating, and the operating point is put back to theinitial state and kept in a range where it can be controlled by theoperating point control circuit. Therefore, the operating point is neverleft in an unstable state.

Then, the LD bank 110 exits light of the wavelength L4. At this time,the output signal of the PD 116 increases to approximately the samelevel as in the case of the wavelength L2. Therefore, the output signalof the PD 116 becomes larger than the reference voltage Vref and hencethe comparator 118 does not send any signals. The switch 119 is turnedon and the LPF 126 is connected to the differential amplifier 127. Atthis time, the operating point control circuit controls the operatingpoint starting from the initial state and hence can operate normally.

While the above description is directed to the case where the LD bank110 stops the light to exit temporally to change the wavelength of exitlight, the operating point can be kept stable in a similar manner alsoin a case where the light exiting is stopped to use another opticaladdition circuit 107 a in the optical add/drop apparatus.

Next, the sixteenth embodiment will be described starting from itsconfiguration.

As shown in FIG. 16, this optical add/drop apparatus is composed ofoptical amplifiers 101 and 103, an OADM 102, an 1×M optical coupler 104,M optical wavelength branching circuits 105, an N×1 opticalmulti/demultiplexer 106, and N optical addition circuits 107 b.

Although this optical add/drop apparatus has the M optical wavelengthbranching circuits 105 and the N optical addition circuits 107 b, inFIG. 16 only one of the M optical wavelength branching circuits 105 andonly one of the N optical addition circuits 107 b are shown by solidlines and the other circuits are shown with broken lines because thecircuits of each group have the same configuration.

A wavelength-division multiplexed optical signal transmitting through anoptical transmission line enters the optical add/drop apparatus and isentered to the OADM 102 via the optical amplifier 101. Signal lightbeams of predetermined wavelengths that have been dropped by the OADM102 are distributed by the 1×M optical coupler 104 and then entered tothe optical wavelength branching circuits 105, where they are received.On the other hand, WDM optical signals to be added in the OADM 102 aregenerated by the optical addition circuits 107 b, which are provided inN, that is, the number of WDM optical signals to be added in the OADM102. The optical signals to be added and an optical signal that has notbeen dropped in the OADM 102 are wavelength-division multiplexed witheach other, and a resulting WDM optical signal is outputted to theoptical transmission line via the optical amplifier 103.

Each optical addition circuit 107 b is composed of an LD bank 110,optical amplifiers 111 and 115, an optical coupler 140, an MZ modulator113, PDs 123 and 141, a buffer amplifier 124, amplifiers 121 and 142, acomparator 143, switches 144 and 148, a variable gain amplifier 120, acoupling capacitor 122, a multiplier 125, an LPF 126, a differentialamplifier 127, an inductor 128, capacitors 129 and 151, resistors 130,145, and 146, a low-frequency oscillator 131, field-effect transistors(hereinafter abbreviated as “FETs”) 147 and 149, and operationalamplifiers 150 and 152.

In FIG. 16, laser light exit from the LD bank 110 is entered to the MZmodulator 113 via the optical amplifier 111.

A modulation signal and a low-frequency signal of a predeterminedfrequency f0 that is outputted from the low-frequency oscillator 131 areinputted to the variable gain amplifier 120. An output signal of thevariable gain amplifier 120 is inputted to one modulation-input terminalof the MZ modulator 113 via the amplifier 121 and the coupling capacitor122.

As for the other modulation-input terminal of the MZ modulator 113, towhich the resistor 130 and a bias T circuit that is composed of theinductor 128 and the capacitor 129 are connected.

The MZ modulator 113 modulates the light of the LD bank, for example, awavelength of L2, with the signal supplied from the drive circuit, intoan optical signal, and outputs it.

The output light of the MZ modulator 113 is branched into three beams bythe optical coupler 140. First branched output light is entered to thePD 123. Second branched output light is entered to the PD 141. Thirdbranched output light is entered to the above-mentioned N×1 opticalmulti/demultiplexer 106 via the optical amplifier 115. The firstbranched output light is detected by the PD 123, and a detection signalis inputted to the multiplier 125 via the buffer amplifier 124. Thelow-frequency signal that is outputted from the low-frequency oscillator131 is also inputted to the multiplier 125. The multiplier 125 comparesthe phases of the input signal supplied from the buffer amplifier 124and the low-frequency signal supplied from the low-frequency oscillator131, and outputs a signal in accordance with a phase difference.

The output signal of the multiplier 125 is inputted to the LPF 126. Anoutput of the LPF 126 is inputted to one input terminal of thedifferential amplifier 127 via the switch 144 as well as to thenon-inverting input terminal (+) of the operational amplifier 152. Theother input terminal of the differential amplifier 127 is grounded. Anoutput of the differential amplifier 127 is inputted to the inductor 128of the bias T circuit, and the bias value is variably controlled so asto correct the operating point.

An output of the operational amplifier 152 is inputted to the drainterminal of the FET 147 and the source terminal of the FET 149.

The gate terminal of the FET 147, which is controlled by the switch 148,is connected to a voltage source Vcc via the switch 148. The sourceterminal of the FET 147 is connected to the inverting input terminal (−)of the operational amplifier 152 via the resistor 145 as well as to theinverting input terminal (−) of the operational amplifier 150 via theresistor 146.

The gate terminal of the FET 149, which is controlled by the switch 148,is connected to the voltage source Vcc via the switch 148. The drainterminal of the FET 149 is grounded via the capacitor 151 and connectedto the non-inverting terminal (+) of the operational amplifier 150.

A circuit composed of the operational amplifiers 150 and 152, the FETs147 and 149, the resistors 145 and 146, and the capacitor 151 is aholding circuit for holding the output voltage of the LPF 126.

On the other hand, the second branched output light is detected by thePD 141, which outputs an electrical signal that is in proportion to theaverage intensity of the second branched output light. That is, the PD141 detects the intensity of the light that is exit from the LD bank 110by monitoring the output light of the MZ modulator 113.

The electrical signal that is outputted from the PD 141 is amplified bythe amplifier 142 and then compared with a reference voltage Vref by thecomparator 143. When the electrical signal is smaller than or equal tothe reference voltage Vref, the comparator 143 outputs a signal to theswitches 144 and 148 and thereby controls these switches.

The switch 144 can switch connecting the LPF 126 to the differentialamplifier 127 and connecting the output terminal of the operationalamplifier 150 to the differential amplifier 127. Usually, the switch 144connects the LPF 126 to the differential amplifier 127, but uponreception of a signal from the comparator 143, the switch 144 switchesto connecting the output terminal of the operational amplifier 150 tothe differential amplifier 127. When it no longer receives the signalcoming from the comparator 143, the switch 144 again connects the LPF126 to the differential amplifier 127.

The switch 148 controls the on/off of the FETs 147 and 149 in accordancewith a signal supplied from the comparator 143. That is, while a signalfrom the comparator 143 is not received, the switch 148 connects thevoltage source Vcc to the gate terminal of the FET 149, thereby keepingthe FET 149 on and keeping the FET 147 off. On the other hand, uponreception of a signal from the comparator 143, the switch 148 turns offthe FET 149 and connects the voltage source Vcc to the gate terminal ofthe FET 147, thereby turning on the FET 147.

Next, functions and advantageous effects of the sixteenth embodimentwill be described.

The optical add/drop apparatus having the above configuration can keepthe operating point of the MZ modulator 113 stable even if the inputlight no longer exists during a period when the wavelength of light exitfrom the LD bank 110 is changed in the optical addition circuit 107 b,for example, during a period laser light of a wavelength L2 is changedto laser light of a wavelength L4.

This will be explained below in the case when the wavelength L2 ischanged to the wavelength L4.

At first, since a wavelength-division multiplexed signal transmittingthrough the optical transmission line has an available wavelength L2,the LD bank 110 exits light of the wavelength L2. The exit light ismodulated by the MZ modulator 113, added by the OADM 102 as additionlight via the N×1 optical multi/demultiplexer 106, and outputted to theoptical transmission line. The exit light is also entered, via theoptical modulator 113 etc., to the operating point control circuit,where it is used to control the operating point of the MZ modulator 113.The exit light is photoelectrically converted by the PD 141 via the MZmodulator 113 etc. An output signal of the PD 141 is judged by thecomparator 143 as to whether it is smaller than or equal to thereference voltage Vref. That is, whether the intensity of the light ofthe wavelength L2 exit from the LD bank 110 is a predetermined value orless can be judged by the comparator 143 as to whether the electricalsignal that is outputted from the PD 141 is smaller than or equal to thepredetermined reference voltage Vref.

Since the light of the wavelength L2 exit from the LD bank 110 is usedas addition light, its light intensity is larger than the predeterminedvalue and hence the comparator 143 does not output any signals.Therefore, the switch 144 keeps connecting the LPF 126 to thedifferential amplifier 127. As a result, the operating point controlcircuit continues to operate normally. Further, the switch 148 turns theFETs 147 and 149 on and off, respectively. As a result, the outputvoltage of the LPF 126 is stored in the capacitor 151.

Then, the available wavelength of a wavelength-division multiplexedsignal transmitting through the optical transmission line is changedfrom L2 to L4, whereupon the LD bank 110 stops exiting the light of thewavelength L2.

At this time, the level of the output signal of the PD 141 decreases toapproximately zero. Since the output signal is smaller than thereference voltage Vref, the comparator 143 sends a signal to theswitches 144 and 148. The switch 144 switches from connecting the LPF126 to the differential amplifier 127 to connecting the output terminalof the operational amplifier 150 to the differential amplifier 127.Further, the switch 148 turns the FET 147 on and turns the FET 149 off.Therefore, the output voltage of the LPF 126 which is as same as thevoltage stored in the capacitor 151 is outputted to the output terminalof the operational amplifier 150. As a result, the differentialamplifier 127 maintains the state just before the LD bank 110 stopsexiting the light of the wavelength L2. Therefore, the operating pointis never in an unstable state.

Then, the LD bank 110 exits light of the wavelength L4. At this time,the output signal of the PD 141 increases to approximately the samelevel as in the case of the wavelength L2. Therefore, the output signalof the PD 141 becomes larger than the reference voltage Vref and hencethe comparator 143 does not send any signals. The switch 144 switchesagain from connecting the output terminal of the operational amplifier150 with the differential amplifier 127 to connecting the LPF 126 withthe differential amplifier 127. Therefore, the operating point controlcircuit controls normally the operating point of the MZ modulator 113based on the optical signal entered from the optical modulator 113.

In addition, since the operating point control circuit holds the statejust before switching the laser light of the wavelength L2 to the laserlight of the wavelength L4, the operating point can be compensated formore quickly than in a case where the control of the operating point isstarted from the initial state.

While the above description is directed to the case where the LD bank110 temporally stops the light to exit to change the wavelength of exitlight, the operating point can be kept stable in a similar manner alsoin a case where the light exiting is stopped to use another opticaladdition circuit 107 b in the optical add/drop apparatus.

Next, the seventeenth embodiment will be described.

At first the seventeenth embodiment will be described starting from itsconfiguration.

This optical add/drop apparatus is composed of optical amplifiers 101and 103, an OADM 102, an 1×M optical coupler 104, M optical wavelengthbranching circuits 105, an N×1 optical multi/demultiplexer 106, and Noptical addition circuits 107 c.

Although this optical add/drop apparatus has the M optical wavelengthbranching circuits 105 and the N optical addition circuits 107 a, inFIG. 17 only one of the M optical wavelength branching circuits 105 andonly one of the N optical addition circuits 107 c are shown by solidlines and the other circuits are shown with broken lines because thecircuits of each group have the same configuration.

A wavelength-division multiplexed optical signal transmitting through anoptical transmission line enters the optical add/drop apparatus, and isamplified by the amplifier 101 and then entered to the OADM 102. Signallight beams of predetermined wavelengths that have been dropped by theOADM 102 are entered to the 1×M optical coupler 104. The opticalsignal(s) distributed by the 1×M optical coupler 104 are entered to theoptical wavelength branching circuits 105, where they are subjected toreceiving operations. On the other hand, optical signals to be added bythe OADM 102 are generated by the optical addition circuits 107 c. Theoptical signals to be added and an optical signal that has not beendropped in the OADM 102 are wavelength-division multiplexed with eachother, and is amplified by the optical amplifier 103 and then outputtedto the optical transmission line.

Each optical addition circuit 107 c is composed of an LD bank 110,optical amplifiers 111 and 115, an optical coupler 114, an MZ modulator113, a PD 123, a diode 160, amplifiers 121 and 162, a buffer amplifier124, a comparator 163, a switch 164, a variable gain amplifier 120, acoupling capacitor 122, a multiplier 125, an LPF 126, a differentialamplifier 127, an inductor 128, a capacitor 129, resistors 130 and 161,and a low-frequency oscillator 131.

In FIG. 17, laser light exit from the LD bank 110 is entered to the MZmodulator 113 via the optical amplifier 111.

A modulation signal and a low-frequency signal of a predeterminedfrequency f0 that is outputted from the low-frequency oscillator 131 areinputted to the variable gain amplifier 120. An output signal of thevariable gain amplifier 120 is inputted to one modulation-input terminalof the MZ modulator 113 via the amplifier 121 and the coupling capacitor122.

The resistor 130 and a bias T circuit that is composed of the inductor128 and the capacitor 129 are connected to the other modulation-inputterminal of the MZ modulator 113.

The MZ modulator 113 modulates the light of the wavelength L2 that isexit from the LD bank 110 into an optical signal with the signalsupplied from the drive circuit and outputs it.

Part of the output light of the MZ modulator 113 is branched off by theoptical coupler 114 and thereby taken out. The other part of the outputlight is entered to the above-mentioned N×1 optical multi/demultiplexer106 via the optical amplifier 115. The branched part of the output lightis detected by the PD 123, and a detection signal is inputted to themultiplier 125 via the buffer amplifier 124. The low-frequency signalthat is outputted from the low-frequency oscillator 131 is also inputtedto the multiplier 125. The multiplier 125 compares the phases of theinput signal supplied from the buffer amplifier 124 and thelow-frequency signal supplied from the low-frequency oscillator 131, andoutputs a signal in accordance with a phase difference.

The output signal of the multiplier 125 is inputted to one inputterminal of the differential amplifier 127 via the LPF 126 and theswitch 164. The other input terminal of the differential amplifier 127is grounded. An output of the differential amplifier 127 is inputted tothe inductor 128 of the bias T circuit, and the bias value is variablycontrolled so as to correct the operating point.

On the other hand, the modulation signal is connected to one terminal ofthe diode 160. The other terminal of the diode 160 is grounded via theresistor 161. The modulation signal is half-wave-rectified by the diode160, whereby a voltage corresponding to the intensity of the modulationsignal is detected at both ends of the resistor 161.

The voltage corresponding to the intensity of the modulation signal isamplified by the amplifier 162 and then compared with a referencevoltage Vref by the comparator 163. If this voltage is smaller than orequal to the reference voltage Vref, the comparator 163 outputs a signalto the switch 164 and controls it.

The switch 164 can switch between connecting the LPF 126 to thedifferential amplifier 127 and connecting a reference voltage V1 to thedifferential amplifier 127. Normally, the switch 164 connects the LPF126 to the differential amplifier 127. Upon reception of a signal fromthe comparator 163, the switch 164 switched to connecting the referencevoltage V1 to the differential amplifier 127. When the signal comingfrom the comparator 163 is terminated, the switch 164 again connects theLPF 126 to the differential amplifier 127.

The reference voltage V1 has a value in a range where the operatingpoint can be controlled by the operating point control circuit.

Next, functions and advantageous effects of the seventeenth embodimentwill be described.

The optical add/drop apparatus having the above configuration can keepthe operating point stable even during a period when there is nomodulation signal to be transmitted in the optical addition circuit 107c.

For example, the optical addition circuit 107 c operates in thefollowing manner in a case where a modulation signal first exists, thenloses its existence, and back in existence again.

At first, a modulation signal to be transmitted modulates input lightthat is supplied from the LD bank 110 with the MZ modulator 113.Modulated input light as addition light is added as addition light bythe OADM 102 supplied via the N×1 optical multi/demultiplexer 106 andoutputted to the optical transmission line.

The signal intensity of the modulation light is detected by the diode160 and the resistor 161, and the comparator 163 judges whether avoltage corresponding to the intensity of the modulation signal issmaller than or equal to the predetermined reference voltage Vref. Thatis, it is judged whether the intensity of the modulation signal is thepredetermined value or less.

At this point, since there exists a modulation signal to be transmitted,the comparator 163 does not send any signals to the switch 164.Therefore, the switch 164 connects the LPF 126 to the differentialamplifier 127. The operation point control circuit operates normally,whereby the operating point of the MZ modulator 113 can be controlled byan optical signal entered from the MZ modulator 113.

Then, since the signal to be added does not exist in the opticaladd/drop apparatus, or an optical addition circuit of the N number ofcircuits other than the current optical addition circuit 107 c is used,the modulation signal will no longer exist.

At this time, the voltage value of the resistor 161 decreases toapproximately zero. Since the voltage value is smaller than thereference voltage Vref, the comparator 163 sends a signal to the switch164. The switch 164 switches from connecting the LPF 126 to thedifferential amplifier 127 to connecting the reference voltage V1 to thedifferential amplifier 127. As a result, the operating point controlcircuit maintains the operating point at the reference voltage V1.Therefore, the operating point is never in an unstable state.

Then, a modulation signal to be transmitted generates again, whereupon avoltage is developed in the resistor 161. As a result, since the voltagevalue becomes larger than the reference voltage Vref, the comparator 163does not send any signals. The switch 164 switches connecting thereference voltage V1 to the differential amplifier 127 to connecting theLPF 126 to the differential amplifier 127. Therefore, the operatingpoint control circuit controls normally the operating point of the MZmodulator 113 based on an optical signal entered from the MZ modulator113, shifting from the state of the reference voltage V1.

In this case, if the reference voltage V1 is selected properly inconsideration of the temperature of the MZ modulator 113 in operationand other factors, the operating point can be compensated more quicklythan in the case of starting the operating point control from theinitial state.

Next, the eighteenth embodiment will be described.

At first, the eighteenth embodiment will be described starting from itsconfiguration.

In FIG. 18, this optical add/drop apparatus is composed of opticalamplifiers 101 and 103, an OADM 102, an 1×M optical coupler 104, Moptical wavelength branching circuits 105, an N×1 opticalmulti/demultiplexer 106, and N optical addition circuits 107 d.

Although this optical add/drop apparatus has the M optical wavelengthbranching circuits 105 and the N optical addition circuits 107 d, inFIG. 18 only one of the M optical wavelength branching circuits 105 andonly one of the N optical addition circuits 107 d are shown by solidlines and the other circuits are shown with broken lines because thecircuits of each group have the same configuration.

A wavelength-division multiplexed optical signal transmitting through anoptical transmission line enters the optical add/drop apparatus, isamplified by the amplifier 101, and then entered to the OADM 102. Signallight beams of predetermined wavelengths that have been dropped by theOADM 102 are entered to the 1×M optical coupler 104. The opticalsignal(s) distributed by the 1×M optical coupler 104 are entered to theoptical wavelength branching circuits 105, where they are subjected toreceiving operations. On the other hand, optical signals to be added inthe OADM 102 are generated by the optical addition circuits 107 d. Theoptical signals to be added and an optical signal that has not beendropped in the OADM 102 are wavelength-division multiplexed with eachother, and a resulting optical signal is amplified by the opticalamplifier 103 and then outputted to the optical transmission line.

Each optical addition circuit 107 d is composed of an LD bank 110,optical amplifiers 111 and 115, optical couplers 112 and 114, an MZmodulator 113, PDs 116 and 123, amplifiers 117, 121, and 162, a bufferamplifier 124, comparators 118 and 163, a switch 119, a variable gainamplifier 120, a coupling capacitor 122, a multiplier 125, an LPF 126, adifferential amplifier 127, an inductor 128, a capacitor 129, resistors130 and 161, a low-frequency oscillator 131, a diode 160, an adder 170,and an optical attenuator 171.

In FIG. 18, laser light exit from the LD bank 110 is entered to theoptical amplifier 111. Amplified light is branched into two beams by theoptical coupler 112, and first branched light is entered to the MZmodulator 113 via the optical attenuator 171.

On the other hand, second branched light that has been branched off bythe optical coupler 112 is entered to the PD 116. An electrical signalthat is outputted from the PD 116 is amplified by the amplifier 117 andthen compared with a reference voltage Vref1 by the comparator 118. Whenthe electrical signal is smaller than or equal to the reference voltageVref1, the comparator 118 outputs a signal to the switch 119 and theadder 170.

The switch 119 is controlled in accordance with the output of thecomparator 118. When receiving a signal from the comparator 118, theswitch 119 is turned off and thereby disconnects the LPF 126 from thedifferential amplifier 127. During a period when no signal is receivedfrom the comparator 118, the switch 119 is kept on and thereby connectsthe LPF 126 to the differential amplifier 127.

A modulation signal and a low-frequency signal of a predeterminedfrequency f0 that is outputted from the low-frequency oscillator 131 areinputted to the variable gain amplifier 120. An output signal of thevariable gain amplifier 120 is inputted to one modulation-input terminalof the MZ modulator 113 via the amplifier 121 and the coupling capacitor122.

The resistor 130 and a bias T circuit that is composed of the inductor128 and the capacitor 129 are connected to the other modulation-inputterminal of the MZ modulator 113.

The MZ modulator 113 modulates light that is exited from the LD bank110, for example, the light of a wavelength L2 into an optical signal,with the signal supplied from the drive circuit, and outputs it.

Part of the output light of the MZ modulator 113 is branched off by theoptical coupler 114 and thereby taken out. The other part of the outputlight is entered to the above-mentioned N×1 optical multi/demultiplexer106 via the optical amplifier 115. The branched part of the output lightis detected by the PD 123, and a detected signal is inputted to themultiplier 125 via the buffer amplifier 124. The low-frequency signalthat is outputted from the low-frequency oscillator 131 is also inputtedto the multiplier 125. The multiplier 125 compares the phases of theinput signal supplied from the buffer amplifier 124 and thelow-frequency signal supplied from the low-frequency oscillator 131, andoutputs a signal in accordance with a phase difference.

The output signal of the multiplier 125 is inputted to one inputterminal of the differential amplifier 127 via the LPF 126 and theswitch 119. The other input terminal of the differential amplifier 127is grounded. An output of the differential amplifier 127 is inputted tothe inductor 128 of the bias T circuit, and the bias value is variablycontrolled so as to correct the operating point of the MZ modulator 113.

On the other hand, the modulation signal is grounded via the diode 160and the resistor 161. A voltage corresponding to the intensity of themodulation signal is detected at both terminals of the resistor 161.

The voltage corresponding to the intensity of the modulation signal isinputted, via the amplifier 162, to the comparator 163, where it iscompared with a reference voltage Vref2. If an electrical signal issmaller than or equal to the reference voltage Vref 2, the comparator163 outputs a signal to the adder 170.

The adder 170 ANDs the signal supplied from the comparator 118 and thesignal supplied from the comparator 163 and outputs a result to theoptical attenuator 171. That is, the adder 170 outputs a signal to theoptical attenuator 171 when receiving (a) signal(s) from either or bothcomparators 118 and 163, and only when no signals are received fromeither of the comparators 118 and 163 does it not output any signals tothe optical attenuator 171.

When receiving an output of the adder 170, the optical attenuator 171attenuates the intensity of the input light that is supplied from theoptical coupler 112 to a predetermined intensity. When receiving nooutput from the adder 170, the optical attenuator 171 trajects the inputlight that is supplied from the optical coupler 112 and outputs it tothe MZ modulator 113.

Next, functions and advantageous effects of the eighteenth embodimentwill be described.

The optical add/drop apparatus having the above configuration can keepthe operating point stable even if the input light loses its existenceduring a period when the wavelength of light exit from the LD bank 110is changed in the optical addition circuit 107 d, for example, during aperiod when laser light of a wavelength L2 is changed to laser light ofa wavelength L4. Further, neither ASE nor input light that is notmodulated with a modulation signal is sent to the N×1 opticalmulti/demultiplexer 106 even during a period when the optical additioncircuit 107 d has no modulation signal to be transmitted or there is nolight to be exited from the LD bank 110.

The operation of the operating point control circuit in the fourthembodiment to stabilize the operating point is the same as that in thefirst embodiment and hence is not described here.

The operation in the fourth embodiment to avoid sending ASE or inputlight that is not modulated with a modulation signal to the N×1 opticalmulti/demultiplexer 106 will be described below.

The intensity of a modulation signal is detected by the diode 160 andthe resistor 161. A voltage corresponding to the intensity of themodulation signal is judged by the comparator 163 as to whether thevoltage is smaller than or equal to the predetermined reference voltageVref2, that is, whether the intensity of the modulation signal is thepredetermined value or less.

When there exists a modulation signal to be transmitted, the comparator163 does not send any signals to the adder 170. Therefore, the adder 170does not output any signals to the optical attenuator 171, and hence theMZ modulator 113 modulates the light received with the modulation signaland outputs resulting light.

On the other hand, when the modulation signal no longer exists, thevoltage value of the resistor 161 decreases to approximately zero. Sincethe voltage value becomes smaller than or equal to the reference voltageVref 2, the comparator 163 sends a signal to the adder 170. Therefore,the adder 170 outputs a signal to the optical attenuator 171, whichattenuates the input light to the predetermined light intensity(including zero). Therefore, neither ASE nor input that is not modulatedwith a modulation signal is sent to the N×1 optical multi/demultiplexer106.

The input light exit from the LD bank 110 is photoelectrically convertedby the PD 116. The comparator 118 judges whether an output signal of thePD 116 is smaller than or equal to the reference voltage Vref1. That is,whether or not input light is being exit from the LD bank 110 can bejudged by the comparator 118 as to whether the electrical signal that isoutputted from the PD 116 is smaller than or equal to the predeterminedreference voltage Vref1.

When the LD bank 110 is exiting input light, the light intensity islarger than the predetermined value and hence the comparator 118 doesnot send any signals to the adder 170. Therefore, the adder 170 does notoutput any signals to the optical attenuator 171, and hence the MZmodulator 113 modulates the light received with a modulation signal andoutputs it.

On the other hand, when the LD bank 110 stops exiting input light, theoutput signal of the PD 116 decreases to approximately zero. Since theoutput signal becomes smaller than or equal to the reference voltageVref1, the comparator 118 sends a signal to the adder 170. Therefore,the adder 170 outputs a signal to the optical attenuator 171, whichattenuates ASE that is generated in the optical amplifier 111 etc. tothe predetermined light intensity (including zero). Therefore, ASE isnot sent to the N×1 optical multi/demultiplexer 106.

Naturally, when neither a modulation signal nor input light exists, theadder 170 outputs a signal to the optical attenuator 171 and hence ASEis not sent to the N×1 optical multi/demultiplexer 106.

Next, the nineteenth embodiment will be described.

At first, the nineteenth embodiment will be described starting from itsconfiguration.

In FIG. 19, this optical add/drop apparatus is composed of opticalamplifiers 101 and 103, an OADM 102, an 1×M optical coupler 104, Moptical wavelength branching circuits 105, an N×1 opticalmulti/demultiplexer 106, and N optical addition circuits 107 e.

Although this optical add/drop apparatus has the M optical wavelengthbranching circuits 105 and the N optical addition circuits 107 e, inFIG. 19 only one of the M optical wavelength branching circuits 105 andonly one of the N optical addition circuits 107 e are shown by solidlines and the other circuits are shown with broken lines because thecircuits of each group have the same configuration.

A wavelength-division multiplexed optical signal transmitting an opticaltransmission line enters the optical add/drop apparatus, and isamplified by the amplifier 101 and then entered to the OADM 102. Signallight beams of predetermined wavelengths that have been dropped by theOADM 102 are entered to the 1×M optical coupler 104. The optical signalsdistributed by the 1×M optical coupler 104 are entered to the opticalwavelength branching circuits 105, where they are subjected to receivingoperations. On the other hand, optical signals to be added in the OADM102 are generated by the optical addition circuits 107 e. The opticalsignals to be added and an optical signal that has not been dropped inthe OADM 102 are wavelength-division multiplexed with each other, and aresulting optical signal is amplified by the optical amplifier 103 andthen outputted to the optical transmission line.

Each optical addition circuit 107 e is composed of an LD bank 110,optical amplifiers 111 and 115, optical couplers 112 and 114, an MZmodulator 113, PDs 116 and 123, amplifiers 117, 121, and 162, a bufferamplifier 124, comparators 118 and 163, a switch 181, a variable gainamplifier 120, a coupling capacitor 122, a multiplier 125, an LPF 126, adifferential amplifier 127, an inductor 128, a capacitor 129, resistors130 and 161, a low-frequency oscillator 131, a diode 160, and an adder180.

In FIG. 19, laser light exit from the LD bank 110 is entered to theoptical amplifier 111. Amplified light is branched into two beams by theoptical coupler 112, and first branched light is entered to the MZmodulator 113.

On the other hand, second branched light that has been branched off bythe optical coupler 112 is entered to the PD 116. An electrical signalthat is outputted from the PD 116 is amplified by the amplifier 117 andthen compared with a reference voltage Vref1 by the comparator 118. Whenthe electrical signal is smaller than or equal to the reference voltageVref1, the comparator 118 outputs a signal to the switch 181 and theadder 180.

The switch 181 can switch between connecting the LPF 126 to thedifferential amplifier 127 and connecting a reference voltage V1 to thedifferential amplifier 127. Normally, the switch 181 connects the LPF126 to the differential amplifier 127. Upon reception of a signal fromthe comparator 118, the switch 181 switches to connecting the referencevoltage V1 to the differential amplifier 127. When the signal comingfrom the comparator 118 is terminated, the switch 181 again connects theLPF 126 to the differential amplifier 127.

The reference voltage V1 has a voltage value in a range where theoperating point can be controlled by the operating point controlcircuit.

A modulation signal and a low-frequency signal of a predeterminedfrequency f0 that is outputted from the low-frequency oscillator 131 areinputted to the variable gain amplifier 120. An output signal of thevariable gain amplifier 120 is inputted to one modulation-input terminalof the MZ modulator 113 via the amplifier 121 and the coupling capacitor122.

The resistor 130 and a bias T circuit that is composed of the inductor128 and the capacitor 129 are connected to the other modulation-inputterminal of the MZ modulator 113.

The MZ modulator 113 modulates the light of a wavelength L2, forexample, that is exit from the LD bank 110 with the signal supplied fromthe drive circuit, into an optical signal, and outputs it. Further, whenreceiving a signal from the adder 180, the MZ modulator 113 is preventedfrom producing output light by shifting the phases of light beamstransmitting through two respective optical waveguides in the MZmodulator 113 to form a phase difference of 180°.

Part of the output light of the MZ modulator 113 is branched off by theoptical coupler 114 and thereby taken out. The other part of the outputlight is entered to the above-mentioned N×1 optical multi/demultiplexer106 via the optical amplifier 115. A part of the branched output lightis detected by the PD 123, and the detected signal is inputted to themultiplier 125 via the buffer amplifier 124. The low-frequency signalthat is outputted from the low-frequency oscillator 131 is also inputtedto the multiplier 125. The multiplier 125 compares the phases of theinput signal supplied from the buffer amplifier 124 and thelow-frequency signal supplied from the low-frequency oscillator 131, andoutputs a signal in accordance with a phase difference.

The output signal of the multiplier 125 is inputted to one inputterminal of the differential amplifier 127 via the LPF 126 and theswitch 181. The other input terminal of the differential amplifier 127is grounded. An output of the differential amplifier 127 is inputted tothe inductor 128 of the bias T circuit, and the bias value is variablycontrolled so as to correct the operating point of the MZ modulator 113.

On the other hand, the modulation signal is grounded via the diode 160and the resistor 161. A voltage corresponding to the intensity of themodulation signal is detected at both ends of the resistor 161.

The voltage corresponding to the intensity of the modulation signal isinputted, via the amplifier 162, to the comparator 163, where it iscompared with a reference voltage Vref2. If this voltage is smaller thanor equal to the reference voltage Vref2, the comparator 163 outputs asignal to the adder 180.

The adder 180 ANDs the outputs of the comparators 118 and 163 andoutputs a result to the MZ modulator 113.

Next, functions and advantageous effects of the nineteenth embodimentwill be described.

The optical add/drop apparatus having the above configuration can keepthe operating point stable even if the input light loses its existenceduring a period when the wavelength of light exit from the LD bank 110is changed in the optical addition circuit 107 e, for example, during aperiod when laser light of a wavelength L2 is changed to laser light ofa wavelength L4. Further, neither ASE nor input light that is notmodulated with a modulation signal is outputted to the N×1 opticalmulti/demultiplexer 106 even during a period when the optical additioncircuit 107 e has no modulation signal to be transmitted or there is nolight to be exited from the LD bank 110.

The operation of the operating point control circuit in the fifthembodiment to stabilize the operating point is the same as that in thefirst embodiment except that, in place of the switch 119 being turned onor off, the switch 164 connects one input terminal of the differentialamplifier 127 to the LPF 126 or the terminal of the reference voltageV1, and hence it is not described here.

The operation in the fifth embodiment to avoid outputting ASE or inputlight that is not modulated with a modulation signal to the N×1 opticalmulti/demultiplexer 106 will be described below.

The intensity of a modulation signal is detected by the diode 160 andthe resistor 161. A voltage corresponding to the intensity of themodulation signal is judged by the comparator 163 as to whether thevoltage is smaller than or equal to the predetermined reference voltageVref2, that is, whether the intensity of the modulation signal is thepredetermined value or less.

When there is a modulation signal to be transmitted, the comparator 163does not output any signals to the adder 180. Therefore, the adder 180does not output any signals to the optical modulator 113, and hence theMZ modulator 113 modulates the received light by the modulation signaland outputs it.

On the other hand, when the modulation signal no longer exists, thevoltage value of the resistor 161 decreases to approximately zero. Sincethe voltage value becomes smaller than or equal to the reference voltageVref 2, the comparator 163 outputs a signal to the adder 180. Therefore,the adder 180 outputs a signal to the optical modulator 113, whichshifts the phase differences of respective light beams transmittingthrough two light waveguides in the MZ modulator 113 by 180°, so theproducing of output light is stopped. Therefore, neither ASE nor inputthat is not modulated with a modulation signal is sent to the N×1optical multi/demultiplexer 106.

The input light exited from the LD bank 110 is photoelectricallyconverted by the PD 116. The comparator 118 judges whether an outputsignal of the PD 116 is smaller than or equal to the reference voltageVref1. That is, whether input light is being exited from the LD bank 110can be judged by the comparator 118 as to whether the electrical signalthat is outputted from the PD 116 is smaller than or equal to thepredetermined reference voltage Vref1.

When the LD bank 110 is exiting input light, the light intensity islarger than the predetermined value and hence the comparator 118 doesnot output any signals to the adder 180. Therefore, the adder 180 doesnot output any signals to the optical modulator 113, and hence the MZmodulator 113 modulates the input light with a modulation signal andoutputs it.

On the other hand, when the LD bank 110 is not exiting light, the outputsignal of the PD 116 decreases to approximately zero. Since the outputsignal becomes smaller than or equal to the reference voltage Vref1, thecomparator 118 outputs a signal to the adder 180. Therefore, the adder180 outputs a signal to the optical modulator 113, which attenuates ASEthat is generated in the optical amplifier 111 and etc. Therefore, ASEis not outputted to the N×1 optical multi/demultiplexer 106.

Naturally, when neither a modulation signal nor input light exists, theadder 180 outputs a signal to the optical modulator 113 and hence ASE isnot outputted to the N×1 optical multi/demultiplexer 106.

The LD bank 110 used in the first to fifth embodiments can be replacedby a tunable wavelength laser capable of exiting light of an arbitrarywavelength.

1. An optical communication apparatus comprising: optical modulatingmeans for modulating input light in accordance with a modulation signalto be transmitted, and for outputting the modulated light from an outputport of the optical modulating means; and regulating means forregulating the intensity of light which is transmitted through anoptical transmission line from said optical modulation means, whereinsaid regulating means is an optical attenuating means, positioneddownstream of the output port, for attenuating the intensity of themodulated light after being output from the output port in accordancewith the intensity of said modulation signal.
 2. An opticalcommunication apparatus comprising: optical modulating means forreceiving an input light at an input port of the optical modulatingmeans, and for modulating the received input light in accordance with amodulation signal to be transmitted; and regulating means for regulatingthe intensity of light which is transmitted through an opticaltransmission line from said optical modulation means, wherein saidregulating means is an optical attenuating means for attenuating theintensity of the input light before being received at the in put port ofsaid optical modulating means in accordance with the intensity of saidmodulation signal.
 3. An optical communication apparatus comprising: anoptical modulator receiving input light at an input port of the opticalmodulator, and modulating the received input light in accordance with amodulation signal to be transmitted; and a regulator regulatingintensity of light which is transmitted through an optical transmissionline from the optical modulator, wherein the regulator is an opticalattenuator attenuating intensity of the input light before beingreceived at input port in accordance with intensity of the modulationsignal.
 4. An optical communication apparatus comprising: an opticalmodulator modulating input light in accordance with a modulation signalto be transmitted, and outputting the modulated light from an outputport of the optical modulator; and a regulator regulating intensity oflight which is transmitted through an optical transmission line from theoptical modulator, wherein the regulator is an optical attenuator,positioned downstream of the output port, attenuating intensity of lightafter being output from the output port in accordance with intensity ofthe modulation signal.
 5. An apparatus comprising: an optical modulatorreceiving an input light and modulating the received input light inaccordance with a modulation signal; and an attenuator attenuatingintensity of the input light before being received by the opticalmodulator in accordance with intensity of the modulation signal.
 6. Anapparatus as in claim 5, wherein, when the intensity of the modulationsignal is below a predetermined level, the attenuator attenuates theintensity of the input light so that ASE is not output from themodulator.
 7. An apparatus as in claim 5, wherein, when the intensity ofthe modulation signal is below a predetermined level, the attenuatorattenuates the intensity of the input light so that unmodulated inputlight is not output from the modulator.
 8. An apparatus comprising: anoptical modulator receiving an input light and modulating the receivedinput light in accordance with a modulation signal; and means forattenuating intensity of the input light before being received by theoptical modulator in accordance with intensity of the modulation signal.9. An apparatus comprising: an optical modulator modulating an inputlight in accordance with a modulation signal, to thereby output amodulated light; and an attenuator, positioned downstream of the opticalmodulator, attenuating intensity of the modulated light output from theoptical modulator in accordance with intensity of the modulation signal.10. An apparatus as in claim 9, wherein, when the intensity of themodulation signal is below a predetermined level, the attenuatorattenuates the intensity of the modulated output light so that ASE isnot output from the modulator to a downstream transmission line.
 11. Anapparatus as in claim 9, wherein, when the intensity of the modulationsignal is below a predetermined level, the attenuator attenuates theintensity of the modulated output light so that unmodulated input lightis not output from the modulator to a downstream transmission line. 12.An apparatus comprising: an optical modulator modulating an input lightin accordance with a modulation signal, to thereby output a modulatedlight; and means, positioned downstream of the optical modulator, forattenuating intensity of the modulated light after being output from theoptical modulator in accordance with intensity of the modulation signal.13. A method comprising: optically modulating an input light inaccordance with a modulation signal; and attenuating intensity of theinput light, before the input light is optically modulated by saidoptically modulating, in accordance with intensity of the modulationsignal.
 14. A method as in claim 13, wherein, when the intensity of themodulation signal is below a predetermined level, said attenuatingattenuates the intensity of the input light so that ASE is not outputfrom said optically modulating.
 15. An apparatus as in claim 13,wherein, when the intensity of the modulation signal is below apredetermined level, said attenuating attenuates the intensity of theinput light so that unmodulated input light is not output from saidoptically modulating.
 16. A method comprising: optically modulating aninput light in accordance with a modulation signal, to thereby output amodulated light; and attenuating, at a point downstream of said opticalmodulating, intensity of the modulated light output from said opticalmodulating in accordance with intensity of the modulation signal.
 17. Amethod as in claim 16, wherein, when the intensity of the modulationsignal is below a predetermined level, said attenuating attenuates theintensity of the modulated output light so that ASE is not output fromsaid optically modulating to a downstream transmission line.
 18. Anapparatus as in claim 16, wherein, when the intensity of the modulationsignal is below a predetermined level, said attenuating attenuates theintensity of the modulated output light so that unmodulated input lightis not output from said optically modulating to a downstreamtransmission line.
 19. An apparatus comprising: an optical modulatormodulating an input light in accordance with a modulation signal; and anattenuator attenuating intensity of the input light in accordance withintensity of the modulation signal, wherein, when the intensity of themodulation signal is below a predetermined level, the attenuatorattenuates the intensity of the input light so that ASE is not outputfrom the modulator.
 20. An apparatus comprising: an optical modulatormodulating an input light in accordance with a modulation signal; and anattenuator attenuating intensity of the input light in accordance withintensity of the modulation signal, wherein, when the intensity of themodulation signal is below a predetermined level, the attenuatorattenuates the intensity of the input light so that unmodulated inputlight is not output from the modulator.
 21. An apparatus comprising: anoptical modulator modulating an input light in accordance with amodulation signal, to thereby output a modulated light; and anattenuator attenuating intensity of the modulated light output from theoptical modulator in accordance with intensity of the modulation signal,wherein, when the intensity of the modulation signal is below apredetermined level, the attenuator attenuates the intensity of themodulated output light so that ASE is not output from the modulator to adownstream transmission line.
 22. An apparatus comprising: an opticalmodulator modulating an input light in accordance with a modulationsignal, to thereby output a modulated light; and an attenuatorattenuating intensity of the modulated light output from the opticalmodulator in accordance with intensity of the modulation signal,wherein, when the intensity of the modulation signal is below apredetermined level, the attenuator attenuates the intensity of themodulated output light so that unmodulated input light is not outputfrom the modulator to a downstream transmission line.
 23. A methodcomprising: optically modulating an input light in accordance with amodulation signal; and attenuating intensity of the input light inaccordance with intensity of the modulation signal, wherein, when theintensity of the modulation signal is below a predetermined level, saidattenuating attenuates the intensity of the input light so that ASE isnot output from said optically modulating.
 24. A method comprising:optically modulating an input light in accordance with a modulationsignal; and attenuating intensity of the input light in accordance withintensity of the modulation signal, wherein, when the intensity of themodulation signal is below a predetermined level, said attenuatingattenuates the intensity of the input light so that unmodulated inputlight is not output from said optically modulating.
 25. A methodcomprising: optically modulating an input light in accordance with amodulation signal, to thereby output a modulated light; and attenuatingintensity of the modulated light output from said optical modulating inaccordance with intensity of the modulation signal, wherein, when theintensity of the modulation signal is below a predetermined level, saidattenuating attenuates the intensity of the modulated output light sothat ASE is not output from said optically modulating to a downstreamtransmission line.
 26. A method comprising: optically modulating aninput light in accordance with a modulation signal, to thereby output amodulated light; and attenuating intensity of the modulated light outputfrom said optical modulating in accordance with intensity of themodulation signal, wherein, when the intensity of the modulation signalis below a predetermined level, said attenuating attenuates theintensity of the modulated output light so that unmodulated input lightis not output from said optically modulating to a downstreamtransmission line.
 27. An apparatus comprising: an optical modulatorhaving in input port and an output port, the optical modulator receivinga light at the input port, optically modulating the received light inaccordance with a modulation signal, and outputting the modulated lightfrom the output port; and an attenuator, positioned before the inputport or after the output port, attenuating intensity of the light inaccordance with intensity of the modulation signal.
 28. An apparatus asin claim 27, further comprising: a detector detecting intensity of themodulation signal, wherein the attenuator attenuates the intensity ofthe light in accordance with the intensity of the modulation signal asdetected by the detector.
 29. An apparatus as in claim 27, furthercomprising: means for controlling the attenuator to attenuate theintensity of the light in accordance with the intensity of themodulation signal.
 30. An apparatus as in claim 28, further comprising:means for controlling the attenuator to attenuate the intensity of thelight in accordance with the intensity of the modulation signal asdetected by the detector.
 31. An apparatus comprising: an opticalmodulator having in input port and an output port, the optical modulatorreceiving a light at the input port, optically modulating the receivedlight in accordance with a modulation signal, and outputting themodulated light from the output port; a variable attenuator, positionedbefore the input port to attenuate intensity of the light before beingreceived at the input port, or positioned after the output port toattenuate intensity of the light after being output as modulated lightfrom the output port; a detector detecting intensity of the modulationsignal; and a controller controlling the attenuator to attenuate theintensity of the light in accordance with the detected intensity of themodulation signal.
 32. An apparatus comprising: modulation means, havingin input port and an output port, for receiving a light at the inputport, for optically modulating the received light in accordance with amodulation signal, and for outputting the modulated light from theoutput port; attenuator means, positioned before the input port toattenuate intensity of the light before being received at the inputport, or positioned after the output port to attenuate intensity of thelight after being output as modulated light from the output port; meansfor detecting intensity of the modulation signal; and means forcontrolling the attenuator means to attenuate the intensity of the lightin accordance with the detected intensity of the modulation signal. 33.An apparatus comprising: an optical modulator receiving an input lightand modulating the received input light in accordance with a modulationsignal; a detector detecting intensity of the modulation signal; and anattenuator attenuating intensity of the input light before beingreceived by the optical modulator in accordance with the detectedintensity of the modulation signal.
 34. An apparatus comprising: anoptical modulator modulating an input light in accordance with amodulation signal, to thereby output a modulated light; a detectordetecting intensity of the modulation signal; and an attenuator,positioned downstream of the optical modulator, attenuating intensity ofthe modulated light output from the optical modulator in accordance withthe detected intensity of the modulation signal.